Added a way to pin diskimagebuilder commit to redstack

We need to have a way to pin the last stable version of diskimage-builder
in case we have an unstable version on trunk that is breaking.

Fixes bug 1175408

Change-Id: Idb02aee0af3d012562c94e44e87f6ac40e0d5460
Merge "Added a way to pin diskimagebuilder commit to redstack"