Add possibility to build iso with specific nailgun/astute/ostf gerrit commits

Some modifications in order to provide support for ISO build with specific nailgun/astute/ostf gerrit commits.
Gerrit URLs and commits can be passed via environment variables.

Change-Id: I32b78c2b850b0f7c9c5d9028cad160eeb9a3b75c
Merge "Add possibility to build iso with specific nailgun/astute/ostf gerrit commits"