samba-tool: add 'import samba.drs_utils' to fsmo.py

On some systems we're seeing this:

 ERROR(<type 'exceptions.AttributeError'>): uncaught exception - 'module' object has no attribute 'drs_utils'
   File "/usr/lib/python2.7/dist-packages/samba/netcmd/__init__.py", line 185, in _run
     return self.run(*args, **kwargs)
   File "/usr/lib/python2.7/dist-packages/samba/netcmd/fsmo.py", line 533, in run
     transfer_dns_role(self.outf, sambaopts, credopts, role, samdb)
   File "/usr/lib/python2.7/dist-packages/samba/netcmd/fsmo.py", line 136, in transfer_dns_role
     except samba.drs_utils.drsException as e:

E.g. it happens on debian stretch (9.9) with python 2.7.13 (on 4.10.4)

While it doesn't happen on ubuntu 18.04 with python 2.7.15rc1 or
with python 3.6.7.

There were also some reports on the mailing lists, see:
https://lists.samba.org/archive/samba-technical/2019-May/133624.html

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13973

Pair-Programmed-With: Stefan Metzmacher <metze@samba.org>

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

Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Thu May 30 08:27:24 UTC 2019 on sn-devel-184

python/samba/netcmd/fsmo.py

@@ -23,6 +23,7 @@ import ldb
 from ldb import LdbError
 from samba.dcerpc import drsuapi, misc
 from samba.auth import system_session
+import samba.drs_utils
 from samba.netcmd import (
     Command,
     CommandError,