fix xpack.usage api name and add a basic REST test for it (elastic/elasticsearch#2308)

There were two api with same name, depending on the platform one or the other was being loaded first, hence the xpack info tests may fail due to unsupported params being used.

Original commit: elastic/x-pack-elasticsearch@bd44eef3cc

elasticsearch/x-pack/src/test/resources/rest-api-spec/api/xpack.info.json

@@ -1,6 +1,6 @@
 {
   "xpack.info": {
-    "documentation": "Retrieve information about xpack, including buid number/timestamp and license status",
+    "documentation": "Retrieve information about xpack, including build number/timestamp and license status",
     "methods": [ "GET" ],
     "url": {
       "path": "/_xpack",

elasticsearch/x-pack/src/test/resources/rest-api-spec/api/xpack.usage.json

@@ -1,5 +1,5 @@
 {
-  "xpack.info": {
+  "xpack.usage": {
     "documentation": "Retrieve information about xpack features usage",
     "methods": [ "GET" ],
     "url": {

elasticsearch/x-pack/src/test/resources/rest-api-spec/test/xpack/15_basic.yaml

@@ -1,6 +1,6 @@
-# Integration tests xpack info API
+# Integration tests xpack info and usage API
 #
-"X-Pack Info":
+"X-Pack Info and Usage":
 
   - do:
       cluster.health:
@@ -83,6 +83,17 @@
   - is_true:    features.monitoring.description
   - is_true:    tagline
 
+  - do:
+    xpack.usage: {}
+  - is_true:    watcher.enabled
+  - is_true:    watcher.available
+  - is_true:    security.enabled
+  - is_true:    security.available
+  - is_true:    graph.enabled
+  - is_true:    graph.available
+  - is_true:    monitoring.enabled
+  - is_true:    monitoring.available
+
   - do:
     xpack.info:
       categories: "_none"