Fix exception when api_extensions is set to empty

test.is_extension_enabled() is not checking if
config_dict[service] list is empty

Added a check if config_dict[service] is empty the
function will return False which means no extensions
are enabled

Change-Id: I2ee64f205c393637b5fc65897f1489292781c0be
Closes-Bug: 1342516

tempest/test.py

@@ -215,6 +215,8 @@ def is_extension_enabled(extension_name, service):
         'network': CONF.network_feature_enabled.api_extensions,
         'object': CONF.object_storage_feature_enabled.discoverable_apis,
     }
+    if len(config_dict[service]) == 0:
+        return False
     if config_dict[service][0] == 'all':
         return True
     if extension_name in config_dict[service]: