selftest: Add test for joining a Samba classic DC as a BDC

selftest: Add test for joining a Samba classic DC as a BDC

This does not join the DC itself, so as not to pertrub the test
environment mid-run, but does confirm that the join works and the
password can be changed.

Pair-programmed-with: Garming Sam <garming@catalyst.net.nz>
Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Thu Dec 11 21:40:27 CET 2014 on sn-devel-104