r25299: Modify the provision script to take an additional argument: --server-role

r25299: Modify the provision script to take an additional argument:  --server-role

This must be set to either 'domain controller', 'domain member' or 'standalone'.

The default for the provision now changes to 'standalone'.

This is not because Samba4 is particularlly useful in that mode, but
because we still want a positive sign from the administrator that we
should advertise as a DC.

We now do more to ensure the 'standalone' and 'member server'
provision output is reasonable, and try not to set odd things into the
database that only belong for the DC.

Andrew Bartlett
(This used to be commit 4cc4ed7719aff712e735628410bd3813c7d6aa40)