_ is a forbidden character for container (repository)

According to [Containers naming guide](http://msdn.microsoft.com/en-us/library/dd135715.aspx):

> A container name must be a valid DNS name, conforming to the following naming rules:
>
> * Container names must start with a letter or number, and can contain only letters, numbers, and the dash (-) character.
> * Every dash (-) character must be immediately preceded and followed by a letter or number; consecutive dashes are not permitted in container names.
> * All letters in a container name must be lowercase.
> * Container names must be from 3 through 63 characters long.

We need to fix the documentation and control that before calling Azure API.
The validation will come with issue #27.

Closes #21.

Author: dadoonet

README.md

@@ -395,9 +395,21 @@ client.admin().cluster().preparePutRepository("my_backup3")
         ).get();
 ```
 
+Repository validation rules
+---------------------------
+
+According to the [containers naming guide](http://msdn.microsoft.com/en-us/library/dd135715.aspx), a container name must 
+be a valid DNS name, conforming to the following naming rules:
+
+* Container names must start with a letter or number, and can contain only letters, numbers, and the dash (-) character.
+* Every dash (-) character must be immediately preceded and followed by a letter or number; consecutive dashes are not 
+permitted in container names.
+* All letters in a container name must be lowercase.
+* Container names must be from 3 through 63 characters long.
+
 
 Testing
--------
+=======
 
 Integrations tests in this plugin require working Azure configuration and therefore disabled by default.
 To enable tests prepare a config file elasticsearch.yml with the following content:

src/test/java/org/elasticsearch/repositories/azure/AzureSnapshotRestoreITest.java

@@ -22,6 +22,7 @@
 
 import com.microsoft.windowsazure.services.core.ServiceException;
 import com.microsoft.windowsazure.services.core.storage.StorageException;
+import org.apache.lucene.util.LuceneTestCase;
 import org.elasticsearch.action.admin.cluster.repositories.put.PutRepositoryResponse;
 import org.elasticsearch.action.admin.cluster.snapshots.create.CreateSnapshotResponse;
 import org.elasticsearch.action.admin.cluster.snapshots.restore.RestoreSnapshotResponse;
@@ -32,6 +33,7 @@
 import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.settings.ImmutableSettings;
 import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.repositories.RepositoryException;
 import org.elasticsearch.repositories.RepositoryMissingException;
 import org.elasticsearch.snapshots.SnapshotState;
 import org.elasticsearch.test.ElasticsearchIntegrationTest;
@@ -44,6 +46,7 @@
 
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.greaterThan;
+import static org.hamcrest.Matchers.is;
 
 /**
  * This test needs Azure to run and -Dtests.azure=true to be set
@@ -224,6 +227,47 @@ public void testMultipleRepositories() {
         assertThat(clusterState.getMetaData().hasIndex("test-idx-2"), equalTo(true));
     }
 
+    /**
+     * For issue #21: https://github.com/elasticsearch/elasticsearch-cloud-azure/issues/21
+     */
+    @Test @LuceneTestCase.AwaitsFix(bugUrl = "https://github.com/elasticsearch/elasticsearch-cloud-azure/issues/21")
+    public void testForbiddenContainerName() {
+        checkContainerName("", false);
+        checkContainerName("es", false);
+        checkContainerName("-elasticsearch", false);
+        checkContainerName("elasticsearch--integration", false);
+        checkContainerName("elasticsearch_integration", false);
+        checkContainerName("ElAsTicsearch_integration", false);
+        checkContainerName("123456789-123456789-123456789-123456789-123456789-123456789-1234", false);
+        checkContainerName("123456789-123456789-123456789-123456789-123456789-123456789-123", true);
+        checkContainerName("elasticsearch-integration", true);
+        checkContainerName("elasticsearch-integration-007", true);
+    }
+
+    /**
+     * Create repository with wrong or correct container name
+     * @param container Container name we want to create
+     * @param correct Is this container name correct
+     */
+    private void checkContainerName(String container, boolean correct) {
+        logger.info("-->  creating azure repository with container name [{}]", container);
+        try {
+            PutRepositoryResponse putRepositoryResponse = client().admin().cluster().preparePutRepository("test-repo")
+                    .setType("azure").setSettings(ImmutableSettings.settingsBuilder()
+                                    .put(AzureStorageService.Fields.CONTAINER, container)
+                                    .put(AzureStorageService.Fields.BASE_PATH, basePath)
+                                    .put(AzureStorageService.Fields.CHUNK_SIZE, randomIntBetween(1000, 10000))
+                    ).get();
+            assertThat(putRepositoryResponse.isAcknowledged(), is(correct));
+            client().admin().cluster().prepareDeleteRepository("test-repo").get();
+        } catch (RepositoryException e) {
+            if (correct) {
+                // We did not expect any exception here :(
+                throw e;
+            }
+        }
+    }
+
     /**
      * Deletes repositories, supports wildcard notation.
      */