Local installation: different base repos

[rattermeyer]

The setup local environment step should allow to specify a company repo as the base for the opendevstack sources as well as a base for the production branch (could be master, production or any other tag).

This allows to locally set-up development environments for companies that make changes to the official code or for testing major refactorings.

relates to opendevstack/opendevstack.github.io#106

[clemensutschig]

@rattermeyer - this was an idea I had for rundeck to allow this ... :)