autobuild: Split backup/restore testenvs out into separate job

autobuild: Split backup/restore testenvs out into separate job

The samba-ad-dc-2 job was reaching its limits with the number of
testenvs and what the resource-limited CI machines can handle.
Samba processes were getting swapped out of memory, causing CI runs
to fail.

This patch splits the backup/restore testenv targets into a separate
autobuild job: samba-ad-dc-backup.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13780

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Tue Feb  5 12:23:31 CET 2019 on sn-devel-144

(cherry picked from commit 95b2c9d7751ae1e5a00e1fb096f045dd73c03d72)