do not modify ACLs/Groups for missing users

do not modify ACLs/Groups for missing users

instead of dropping ACLs and group membership for missing users,
simply warn and leave it in the config

for users that get removed via the api this happens explicitely

this is to prevent that a 'faulty' ldapsync removes users temporarily
and with it all acls that the admin created

we still have a 'purge' flag for the sync where ACLs get removed
explicitly for users removed from ldap

also adapt the tests

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>