Blender Git Loki

Buildbot: rework to support branch selection.

Issue with previous code was that branch selected was applied to all repositories.
To be able to control branch on a per-repo basis, we need to use codebases instead.

This commit only enables branch (master/testbuild) and revision selection for master,
we probably do not need this for submodules currently (easy to enable anyway).

Note: tested on local buildbot setup, for linux only (up to build step, have no build env).