For ldap, API wrongly reports user is in group

When the ldap identity backend is configured,
HEAD v3/groups/​{group_id}​/users/​{user_id}
always returns 200, even if the user is not actually in the group.
This is because the sql and kvs backend will raise NotFound
exception if the user is not in the group, but the ldap backend
just return result.

Change-Id: Ie1585c8aebe054091bd76fded666bf41125ff9ca
Closes-Bug: 1245247