CI: Use PATH to find kolla-ansible script

This change also updates the CI test scripts to use PATH to find the
kolla-ansible script, rather than relying on the file in the source
checkout.

Using the script in the source checkout was hiding an issue with pip
install --user, although that has now been fixed in
I5b47a146627d06bb3fe4a747c5f20290c726b0f9.

Related-Bug: #1915527

Change-Id: I2827a657c8716a9c40391c6bdb7ff1a2a9c1260e
Depends-On: https://review.opendev.org/c/openstack/kolla-ansible/+/775586