Fix object storage capabilities client return value

All service clients methods return their response wrapped in
ResponseBody, ResponseBodyData or ResponseBodyList.

But object storage service clients were left out of this
because response from object storage APIs are not same way other
service return. Some APIs return is as string etc.

This commit makes capabilities_client to return ResponseBody
object with consistency to other service clients.

Also add unit tests for that.

This is step to move these clients to lib.

Partially implements blueprint consistent-service-method-names

Change-Id: Ida85033c06a50011b2cd9a86941089fb1ea7bedd
diff --git a/tempest/cmd/verify_tempest_config.py b/tempest/cmd/verify_tempest_config.py
index 2f4d120..8e71ecc 100644
--- a/tempest/cmd/verify_tempest_config.py
+++ b/tempest/cmd/verify_tempest_config.py
@@ -200,7 +200,7 @@
     if service != 'swift':
         resp = extensions_client.list_extensions()
     else:
-        __, resp = extensions_client.list_capabilities()
+        resp = extensions_client.list_capabilities()
     # For Nova, Cinder and Neutron we use the alias name rather than the
     # 'name' field because the alias is considered to be the canonical
     # name.