CI: Use template-overrides.j2 from kolla

Some kolla-ansible jobs failed due to using external mirrors
instead of local ones.
This was due to not using the template override provided by kolla.
This patch fixes that.

Depends-On: https://review.opendev.org/668226


Change-Id: I27f714fdf05e521aa8ce25c5683a452ceb35eeb8
Signed-off-by: Radosław Piliszek <radoslaw.piliszek@gmail.com>