Extract required-projects from job content

In v2, PROJECTS was fed to zuul-cloner and it would then try to clone
things. That's not how v3 works. In v3, we need required-projects stated
in the job definition so that Zuul can properly clone things.

Extract PROJECTS and DEVSTACK_PROJECT_FROM_GIT from job definitions to
try to attach appropriate required-projects lists.

dsvm jobs still have a larger list this doesn't account for though.

This also removes support for specifying a required-projects list in the
mapping, since we weren't actually using it and the semantics of which
thing should win were confusing.

Change-Id: I01156b38a1e907677f69042a003eb580ba3d224c
1 file changed