The tag-pipeline does not set zuul.branch, so define it to an empty
string if it's unset so that the script passes.
Change-Id: Iddcf722abec5c6a69f40cb109e5563a698ab349c
This is a single task within the existing afs prep role, but we
need to be able to do it alone for certain special doc builds,
like infra index page generation.
We could use include_role in the future to de-duplicate this, once
zuul_json supports that.
Change-Id: Ia6c6257cb1479abef79bf919cb6ad25ea6828d90