Fix setting passwords in pdb_ads

Fix setting passwords in pdb_ads

The samba4 password_hash module does not allow changing the password fields via
the "delete oldval" -> "add newval" set of modify operations, it requires a
single "replace with newval" operation.

Andrew, Samba3 by default uses that delete/add pair to detect if between
fetching the old value and storing the new one the old value has changed. This
is lost by using the "replace" operation.

Would it make sense to add this to the password_hash module?

Volker