Mechanism for Dockerfile customization

This patchset enables potentially full customization of Dockerfiles
provided by Kolla without having to change main Kolla code. Mechanism is
based on jinja2 block mechanism. In our dockerfiles we'll add blocks
(mostly empty) with highly standarized naming, then in override file
users will be able to fill/override contents of these blocks. Example is
shown in this patch.
To enable cutomized override file call build.py

--template-override=path-to-file Only caveat is that header {% extends
.. %} has to stay

Change-Id: I5957024e5652f9677c439fb2fcd524e467b951af
Partially-implements: blueprint third-party-plugin-support