add both v2 and v3 tests for get specified extension

this add tests for getting specified extension info both against
on nova v2 and v3 api.

Partially implements blueprint nova-v3-api-tests

Change-Id: I9e254069db737d3eb9088475d7272d8aa79b1bd1
diff --git a/tempest/services/compute/json/extensions_client.py b/tempest/services/compute/json/extensions_client.py
index ce46a9b..ad5354c 100644
--- a/tempest/services/compute/json/extensions_client.py
+++ b/tempest/services/compute/json/extensions_client.py
@@ -37,3 +37,8 @@
         _, extensions = self.list_extensions()
         exts = extensions['extensions']
         return any([e for e in exts if e['name'] == extension])
+
+    def get_extension(self, extension_alias):
+        resp, body = self.get('extensions/%s' % extension_alias)
+        body = json.loads(body)
+        return resp, body['extension']