Add support for POST requests to SLM Execute API

This commit adds support for POST requests to the SLM `_execute` API,
because POST is a more appropriate HTTP verb for this action as it is
not idempotent. The docs are also changed to favor POST over PUT,
although PUT is not removed or officially deprecated.

Author: gwbrown

docs/reference/ilm/apis/slm-api.asciidoc

@@ -185,7 +185,7 @@ To take an immediate snapshot using a policy, use the following
 
 [source,console]
 --------------------------------------------------
-PUT /_slm/policy/daily-snapshots/_execute
+POST /_slm/policy/daily-snapshots/_execute
 --------------------------------------------------
 // TEST[skip:we can't easily handle snapshots from docs tests]
 
@@ -279,7 +279,7 @@ Another snapshot can immediately be executed to ensure the new policy works:
 
 [source,console]
 --------------------------------------------------
-PUT /_slm/policy/daily-snapshots/_execute
+POST /_slm/policy/daily-snapshots/_execute
 --------------------------------------------------
 // TEST[skip:we can't handle snapshots in docs tests]
 

docs/reference/ilm/getting-started-slm.asciidoc

@@ -132,7 +132,7 @@ as using the configuration from our policy right now instead of waiting for
 
 [source,console]
 --------------------------------------------------
-PUT /_slm/policy/nightly-snapshots/_execute
+POST /_slm/policy/nightly-snapshots/_execute
 --------------------------------------------------
 // TEST[skip:we can't easily handle snapshots from docs tests]
 

x-pack/plugin/ilm/qa/multi-node/src/test/java/org/elasticsearch/xpack/slm/SnapshotLifecycleRestIT.java

@@ -210,7 +210,7 @@ public void testPolicyManualExecution() throws Exception {
         createSnapshotPolicy(policyName, "snap", "1 2 3 4 5 ?", repoId, indexName, true);
 
         ResponseException badResp = expectThrows(ResponseException.class,
-            () -> client().performRequest(new Request("PUT", "/_slm/policy/" + policyName + "-bad/_execute")));
+            () -> client().performRequest(new Request("POST", "/_slm/policy/" + policyName + "-bad/_execute")));
         assertThat(EntityUtils.toString(badResp.getResponse().getEntity()),
             containsString("no such snapshot lifecycle policy [" + policyName + "-bad]"));
 
@@ -338,7 +338,7 @@ public void testBasicTimeBasedRetenion() throws Exception {
      */
     private String executePolicy(String policyId) {
         try {
-            Response executeRepsonse = client().performRequest(new Request("PUT", "/_slm/policy/" + policyId + "/_execute"));
+            Response executeRepsonse = client().performRequest(new Request("POST", "/_slm/policy/" + policyId + "/_execute"));
             try (XContentParser parser = JsonXContent.jsonXContent.createParser(NamedXContentRegistry.EMPTY,
                 DeprecationHandler.THROW_UNSUPPORTED_OPERATION, EntityUtils.toByteArray(executeRepsonse.getEntity()))) {
                 return parser.mapStrings().get("snapshot_name");

x-pack/plugin/ilm/src/main/java/org/elasticsearch/xpack/slm/action/RestExecuteSnapshotLifecycleAction.java

@@ -17,6 +17,7 @@ public class RestExecuteSnapshotLifecycleAction extends BaseRestHandler {
 
     public RestExecuteSnapshotLifecycleAction(RestController controller) {
         controller.registerHandler(RestRequest.Method.PUT, "/_slm/policy/{name}/_execute", this);
+        controller.registerHandler(RestRequest.Method.POST, "/_slm/policy/{name}/_execute", this);
     }
 
     @Override