selftest: offline backup restore target

selftest: offline backup restore target

This is a selftest target built from a restored offline backup.
Other backup routines are modified to remove the assumption that every backup
requires server and credentials arguments, since offline backup doesn't
want them.  Also, prepare_dc_testenv now returns the generated ctx so we can
run or re-run routines that require it later.

Signed-off-by: Aaron Haslett <aaron.haslett@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>
Autobuild-User(master): Gary Lockyer <gary@samba.org>
Autobuild-Date(master): Mon Aug  6 08:45:19 CEST 2018 on sn-devel-144