Support the hints mechanism in list_credentials()

This fix implements the hints mechanism, considering filters as hints,
so the particular backend implementation has an option to process or
ignore it. Since the EC2 credentials code calls list_credentials() with
user_id as a param, a separate method list_credentials_for_user has been
introduced to provide the compatibility while support the standard hints
mechanism in list_credentials().

This fix doesn't plug hints into the controller, it prepares the way for
implementation of the bp filter-credentials-by-user to support filtering
credentials by user in a follow on patch.

Closes-Bug: #1353511

Change-Id: Ibcf59aa45a8fc7e5cc66fd4edb91ae8fdc641d93
Merge "Support the hints mechanism in list_credentials()"