[Test] Add client param indexExists

This allows us to use the admin client to easily check whether an
index exists (that may not be visible to the standard client)

test/framework/src/main/java/org/elasticsearch/test/rest/ESRestTestCase.java

@@ -1898,7 +1898,11 @@ protected Map<String, Object> getIndexMappingAsMap(String index) throws IOExcept
     }
 
     protected static boolean indexExists(String index) throws IOException {
-        Response response = client().performRequest(new Request("HEAD", "/" + index));
+        return indexExists(client(), index);
+    }
+
+    protected static boolean indexExists(RestClient client, String index) throws IOException {
+        Response response = client.performRequest(new Request("HEAD", "/" + index));
         return RestStatus.OK.getStatus() == response.getStatusLine().getStatusCode();
     }