upgradeprovision: Change test to always use 2008 R2 schema

upgradeprovision: Change test to always use 2008 R2 schema

This tool (and the corresponding test) is designed to migrate a Samba DC
from a pre-4.0.0 release up to a more recent schema (i.e. Windows 2008R2).

Going further than 2008R2 turns this test into a bit of a nightmare. We
now have a better adprep/'samba-tool domain schemaupgrade' option for
upgrading from 2008R2 to a more recent schema.

It seems to make most sense to leave this tests just running against
2008R2 schema provisions and add new tests to migrate from 2008R2 to
2012R2.

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