Add human readable version of modified date for snapshot lifecycle policy (#41035)

* Add human readable version of modified date for snapshot lifecycle policy

This small change changes it from:

```
...
"modified_date": 1554843903242,
...
```

To

```
...
"modified_date" : "2019-04-09T21:05:03.242Z",
"modified_date_millis" : 1554843903242,
...
```

Including the `"modified_date"` field when the `?human` field is used.

Relates to #38461

* Fix test

Author: dakrone

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/snapshotlifecycle/SnapshotLifecyclePolicyItem.java

@@ -113,7 +113,7 @@ public boolean equals(Object obj) {
     public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
         builder.startObject(policy.getId());
         builder.field("version", version);
-        builder.field("modified_date", modifiedDate);
+        builder.timeField("modified_date_millis", "modified_date", modifiedDate);
         builder.field("policy", policy);
         if (lastSuccess != null) {
             builder.field("last_success", lastSuccess);

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

@@ -93,7 +93,7 @@ public void testFullPolicySnapshot() throws Exception {
             Map<String, Object> lastSuccessObject = (Map<String, Object>) policyMetadataMap.get("last_success");
             assertNotNull(lastSuccessObject);
             Long lastSuccess = (Long) lastSuccessObject.get("time");
-            Long modifiedDate = (Long) policyMetadataMap.get("modified_date");
+            Long modifiedDate = (Long) policyMetadataMap.get("modified_date_millis");
             assertNotNull(lastSuccess);
             assertNotNull(modifiedDate);
             assertThat(lastSuccess, greaterThan(modifiedDate));
@@ -133,7 +133,7 @@ public void testPolicyFailure() throws Exception {
                 assertNotNull(lastFailureObject);
 
                 Long lastFailure = (Long) lastFailureObject.get("time");
-                Long modifiedDate = (Long) policyMetadataMap.get("modified_date");
+                Long modifiedDate = (Long) policyMetadataMap.get("modified_date_millis");
                 assertNotNull(lastFailure);
                 assertNotNull(modifiedDate);
                 assertThat(lastFailure, greaterThan(modifiedDate));