Fix incorrect argument position in DbQuotaDriver

The prototype of db.quota_get is:
`quota_get(context, project_id, resource, user_id=None)`
However, in DbQuotaDriver::get_by_project_and_user(), the `user_id`
is incorrectly used as a positional argument.

Closes-Bug: #1255035

Change-Id: I5b6a4c59efe6d9bdc8380e50b7f7c1ff0d2029b1
Merge "Fix incorrect argument position in DbQuotaDriver"