provision: Remove --username and --password options from samba-tool domain provision

provision: Remove --username and --password options from samba-tool domain provision

This avoids confusion, because the LDAP backend does not use these,
and they do not set the password for the administrator account either!

This may break support for the 'existing' backend LDAP backend, but
that is nothing more than a stub for future development anyway, and
new work in this area should use EXTERNAL in any case.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>