hostgroup: Reduce addition and deletion of members to changed only

Use gen_add_list and gen_intersection_list for host, hostgroup,
membermanager_user and membermanager_group member handling.

The functions are used to reduce the add lists to the new entries
only and the delete lists to the entries that are in the user and
the show list result.

This enables to remove the ignores for "already a member" and
"not a member" errors..