Renamed the status field in the stored watch

was renamed to `_status` so it'll distinct from the other "formal" fields of the watch and also be aligned with the `_status` field that is returned by the Get API.

Also updated the installation docs

Original commit: elastic/x-pack-elasticsearch@1081b75d13

rest-api-spec/test/ack_watch/10_basic.yaml

@@ -43,7 +43,7 @@
       watcher.ack_watch:
         watch_id: "my_watch"
 
-  - match: { "status.actions.test_index.ack.state" : "awaits_successful_execution" }
+  - match: { "_status.actions.test_index.ack.state" : "awaits_successful_execution" }
 
   - do:
       watcher.delete_watch:

rest-api-spec/test/ack_watch/20_ack_individual_action.yaml

@@ -44,7 +44,7 @@
         watch_id: "my_watch"
         action_id: "test_index"
 
-  - match: { "status.actions.test_index.ack.state" : "awaits_successful_execution" }
+  - match: { "_status.actions.test_index.ack.state" : "awaits_successful_execution" }
 
   - do:
       watcher.delete_watch:

src/main/java/org/elasticsearch/watcher/watch/Watch.java

@@ -358,6 +358,6 @@ public class Watch implements TriggerEngine.Job, ToXContent {
         ParseField TRANSFORM = new ParseField("transform");
         ParseField THROTTLE_PERIOD = new ParseField("throttle_period");
         ParseField METADATA = new ParseField("metadata");
-        ParseField STATUS = new ParseField("status");
+        ParseField STATUS = new ParseField("_status");
     }
 }

src/main/resources/watches.json

@@ -12,6 +12,11 @@
         "enabled" : false
       },
       "properties": {
+        "_status": {
+          "type": "object",
+          "enabled" : false,
+          "dynamic" : true
+        },
         "trigger" : {
           "type": "object",
           "enabled" : false,
@@ -27,11 +32,6 @@
           "enabled" : false,
           "dynamic" : true
         },
-        "status": {
-          "type": "object",
-          "enabled" : false,
-          "dynamic" : true
-        },
         "throttle_period": {
           "type": "string",
           "index" : "no"