Fix broken local and global building

Two things broke in the related patches. This patch aims to fix that.

1) tools/build.py was no longer usable as it imported kolla from the
global pip install source. If that didnt exist (because kolla wasn't
installed with pip) then it breaks on import.

2) When kolla was installed globally it looked up the path that
matched the installation of docker-py instead of looking up a path
that had the stuff we wanted (kolla images directory)

Change-Id: I94f1856547ce54d506de72926b08c966a36ac608
Related-Id: I05f10d13e7ba1e2b985c2944aec71ce55630442b
Related-Bug: #1530256
Closes-Bug: #1531569