Enable filtering of credentials by user ID

A credentials entity has a user_id attribute. Currently the lack of a
filter of user_id means that we cannot use the keystone policy file to
enable users to have access to (only) their credentials. This fix solves
it by adding such a filter:

List credentials: `GET /credentials`
Optional query parameters:
- `user_id` (string)

Implements: blueprint filter-credentials-by-user

Closes-Bug: #1267096

Change-Id: Iff016fac37b50d55d77ec7511aae4e57af34f08f
Merge "Enable filtering of credentials by user ID"