Unimplemented get roles by group for project list

The list_projects_for_user() function on LDAP assignment backend only
listed projects with associations across user_id, not across group_ids.
This function admits the group_ids parameter, but never is used on the
body of the function.

Change-Id: I0ff6791e11aa18ffb3a4407e8e5958ac03f2086b
Closes-Bug: #1284639
Merge "Unimplemented get roles by group for project list"