selftest: Don't run KCC on backup testenvs (to avoid flappiness)

selftest: Don't run KCC on backup testenvs (to avoid flappiness)

KCC onthe backup domain (i.e. backupfromdc, restoredc, offlinebackupdc)
can establish new connections for replication. Depending on timing,
this can cause the join_ldapcmp test to fail, because there's an extra
object under the NTDS Settings, at the point the ldapcmp is done.

We don't need any replication to happen on the backup domain. The
backup/restore workflow in the real world should mean that the restored DC
is never run in the same network as the original DC.

This patch updates the default KCC command for the backup testenvs to be
a no-op, so the DCs won't create new connection objects.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Thu Dec  6 12:03:53 CET 2018 on sn-devel-144