selftest: Add new 2-DC testenv for live schema upgrade

selftest: Add new 2-DC testenv for live schema upgrade

This adds a new 2-DC testenv that:
1. Provisions an AD DC with 2008R2 schema
2. Joins another AD DC with 2008R2 schema
3. Starts Samba
4. Performs a live schema upgrade on the PDC
Testenv targetting in tests.py files for this testenv required that we
extend the environment dependencies system to include optional post-startup
dependencies specified in ENV_DEPS_POST maps.

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