Add monitoring daemon to the masterd group

The hmac.key file is owned by group masterd and is required to communicate with
the confd.

The monitoring daemon need to communicate with the confd so it has to be part of
the masterd group.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>