Low: python: Fix setting schema directory in source checkout.

If running from a source checkout and you don't have
PCMK_schema_directory set, then rng_directory would fail because it
returns an incorrect path. This hasn't failed for me in the past
because even though I run cts tests from a source checkout, I always
have PCMK_schema_directory set locally in /etc/sysconfig/pacemaker.

Fixes T982