Fix context error for user

In this changeset, I fixed an error in the Watcher RequestContext
that was introduced by the release of oslo.context==2.12.X which
makes user_id an alias of user (among others)
https://github.com/openstack/oslo.context/blob/2.12.1/oslo_context/context.py#L258-L269

Change-Id: I4e35c675de7ee01db90f700eece0b28413b34c47

watcher/common/context.py

@@ -75,7 +75,7 @@ class RequestContext(context.RequestContext):
         self.domain_name = domain_name
         self.domain_id = domain_id
         self.auth_token_info = auth_token_info
-        self.user_id = user_id
+        self.user_id = user_id or user
         self.project_id = project_id
         if not timestamp:
             timestamp = timeutils.utcnow()