allow for generic branch overrides

This allows us to specify project specific branch overrides in
jjb definitions. For instance, OVERRIDE_TEMPEST_PROJECT_BRANCH=master
to always test with tempest master.

Now with support for projects like oslo.foo

Change-Id: Ic259c5b60a307b87771514849a6a78eb1bc32c75
Merge "allow for generic branch overrides"