domain.py: Add base-schema option to samba-tool provision

domain.py: Add base-schema option to samba-tool provision

Allow a different base-schema to be used when provisioning a new domain.
This allows us to test the new 2012 schema without committing Samba to
using it by default.

If, in future, we change the default to use the 2012 schema, some
existing Samba tests (like upgradeprovision) rely on the 2012 schema.
So making the base-schema optional allows these tests to continue using
the older schema.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>