r22170: To get the smbclient blackbox test to pass again, we need to get the

r22170: To get the smbclient blackbox test to pass again, we need to get the
private dir to contain a valid machine account.

It isn't really valid it use the DC's account any more, so extend this
script to also join the domain.  This nicely tests out some previously
untested code too!

Andrew Bartlett
(This used to be commit 12f4e6033ed11a010211a4295424588f44f7e5e0)