Improve offline build scenarios

[Gaël THEROND]

**Bug Report**

What happened:

When building kolla images from an offline environment (a.k.a No internet access in here), you'll get few build errors even when using a binary installation scenario as few roles rely on retrieving archives/tarballs or even just requirements file from internet.

For most of those roles, they're embedded on templating blocks that can be overrided through template-override.j2 template file.

The issue here is you can quickly end up with a really long and cluttered override file.

What you expected to happen:
We should be able to build kolla images from an offline environment with as little blocks overrides as possible.

How to reproduce it (minimal and precise):
Try to build CentOS or Ubuntu based using binary approach without internet access.