[Trivial] Fix swift's extension discovery

In case the swift's endpoint returned an exception during an
extension query, get_capabilities method returned a string of
an empty dict, which is always "true" and therefore it confues
a later if statement checking if any extensions were found.
Let's make the method return an object - dict - which will
be either true (if contains any data) or false (if it's empty).

Change-Id: Ie7974c97a4b119e41fba7150bcd2a2553300f694

config_tempest/services/object_storage.py

@@ -32,7 +32,6 @@ class ObjectStorageService(Service):
                 # (e.g.: when ceph is on swift)
                 capabilities = self.get_capabilities('swift/info')
 
-            capabilities = json.loads(capabilities)
             # Remove Swift general information from extensions list
             capabilities.pop('swift', {})
             self.extensions = capabilities.keys()
@@ -48,7 +47,7 @@ class ObjectStorageService(Service):
             LOG.warning('Object storage %s API endpoint not discovered. '
                         'Error message: %s', path, e)
 
-        return body
+        return json.loads(body)
 
     def list_create_roles(self, conf, client):
         try: