validation: Use blucon.sh if python3 not available

The Validation project supports running the test suites using nothing
more than Docker as a prerequisite (i.e. if no python3 is available),
so leverage `blucon.sh` now that it supports overriding the validation
git repository path and results dir path.

For most slaves, the new method is merely a fail-safe, since they
already have python3.

JIRA: VAL-95

Depends-on: I4d3c10881de28c64bcca05ff23aa1025f67a1f5e

Change-Id: I08e89300637862e40344131532ec40efd5f68501
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>