License Plugin: Added the `xpack` namespace to the REST API definition

In order to be consistent with the other X-Pack APIs, the `license.*` APIs
have been prefixed with `xpack`.

Closes elastic/elasticsearch#2777

Original commit: elastic/x-pack-elasticsearch@cd35a2d4aa

elasticsearch/x-pack/license-plugin/src/test/resources/rest-api-spec/api/xpack.license.delete.json

@@ -1,5 +1,5 @@
 {
-  "license.delete": {
+  "xpack.license.delete": {
     "documentation": "https://www.elastic.co/guide/en/shield/current/license-management.html",
     "methods": ["DELETE"],
     "url": {

elasticsearch/x-pack/license-plugin/src/test/resources/rest-api-spec/api/xpack.license.get.json

@@ -1,5 +1,5 @@
 {
-  "license.get": {
+  "xpack.license.get": {
     "documentation": "https://www.elastic.co/guide/en/shield/current/license-management.html",
     "methods": ["GET"],
     "url": {

elasticsearch/x-pack/license-plugin/src/test/resources/rest-api-spec/api/xpack.license.post.json

@@ -1,5 +1,5 @@
 {
-  "license.post": {
+  "xpack.license.post": {
     "documentation": "https://www.elastic.co/guide/en/shield/current/license-management.html",
     "methods": ["PUT", "POST"],
     "url": {