SQL fix for get_roles_for_user_and_project user=group ID

When there was a role assigned to a group with the same ID as a
user, the SQL assignment backend would incorrectly return the
assignment to the group when requesting roles for the user via
the get_roles_for_user_and_project method.

With this change, assignments to a group with the same ID are not
returned for the user when calling get_roles_for_user_and_project.

Change-Id: Id3d6f66c995e65e37d909359420d71ecdde86b69
Partial-Bug: #1309228