Migrated extensions_client.py from tempest

This migrates the above files from tempest. This includes tempest commits:

 * extensions_client.py     : I8dcb70ce6ba2af59c7636c5f83157dd058fb1df3
 * test_extensions_client.py: I3ad6761651cec5e66012d08e6b63322f53286a5c
 * extensions.py            : Ica929c402a3d042ae751302384e68853eb28b405

to see the commit history for these files refer to the above Change-Ids
in the tempest repository.

Partially implements blueprint migrate-service-clients-to-tempest-lib

Change-Id: Ic658f09649ed9498fe6d3e7792a7c370772f165b

tempest_lib/services/compute/extensions_client.py

@@ -0,0 +1,34 @@
+# Copyright 2012 OpenStack Foundation
+# All Rights Reserved.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+from oslo_serialization import jsonutils as json
+
+from tempest_lib.api_schema.response.compute.v2_1 import extensions as schema
+from tempest_lib.common import rest_client
+
+
+class ExtensionsClient(rest_client.RestClient):
+
+    def list_extensions(self):
+        url = 'extensions'
+        resp, body = self.get(url)
+        body = json.loads(body)
+        self.validate_response(schema.list_extensions, resp, body)
+        return rest_client.ResponseBody(resp, body)
+
+    def show_extension(self, extension_alias):
+        resp, body = self.get('extensions/%s' % extension_alias)
+        body = json.loads(body)
+        return rest_client.ResponseBody(resp, body)