Allow disabling of sigar via settings

Sigar can only be disabled by removing the binaries. This is tricky for our
tests and might cause a lot of trouble if a user wants or needs to do it.
This commit allows to disable sigar with a simple boolean flag in the settings.

Closes #9582

Conflicts:
	src/main/java/org/elasticsearch/bootstrap/Bootstrap.java
	src/test/java/org/elasticsearch/bootstrap/BootstrapForTesting.java
	src/test/java/org/elasticsearch/test/InternalTestCluster.java

Author: s1monw

src/main/java/org/elasticsearch/monitor/sigar/SigarService.java

@@ -34,22 +34,23 @@ public class SigarService extends AbstractComponent {
     @Inject
     public SigarService(Settings settings) {
         super(settings);
-
         Sigar sigar = null;
-        try {
-            sigar = new Sigar();
-            // call it to make sure the library was loaded
-            sigar.getPid();
-            logger.trace("sigar loaded successfully");
-        } catch (Throwable t) {
-            logger.trace("failed to load sigar", t);
-            if (sigar != null) {
-                try {
-                    sigar.close();
-                } catch (Throwable t1) {
-                    // ignore
-                } finally {
-                    sigar = null;
+        if (settings.getAsBoolean("bootstrap.sigar", true)) {
+            try {
+                sigar = new Sigar();
+                // call it to make sure the library was loaded
+                sigar.getPid();
+                logger.trace("sigar loaded successfully");
+            } catch (Throwable t) {
+                logger.trace("failed to load sigar", t);
+                if (sigar != null) {
+                    try {
+                        sigar.close();
+                    } catch (Throwable t1) {
+                        // ignore
+                    } finally {
+                        sigar = null;
+                    }
                 }
             }
         }

src/test/java/org/elasticsearch/test/InternalTestCluster.java

@@ -88,6 +88,7 @@
 import org.elasticsearch.indices.breaker.HierarchyCircuitBreakerService;
 import org.elasticsearch.indices.fielddata.cache.IndicesFieldDataCache;
 import org.elasticsearch.indices.recovery.RecoverySettings;
+import org.elasticsearch.monitor.sigar.SigarService;
 import org.elasticsearch.node.Node;
 import org.elasticsearch.node.internal.InternalNode;
 import org.elasticsearch.node.service.NodeService;
@@ -274,6 +275,7 @@ public InternalTestCluster(long clusterSeed,
                 builder.put("path.data", dataPath.toString());
             }
         }
+        builder.put("bootstrap.sigar", rarely());
         final int basePort = 9300 + (100 * (jvmOrdinal+1));
         builder.put("transport.tcp.port", basePort + "-" + (basePort+100));
         builder.put("http.port", basePort+101 + "-" + (basePort+200));