workunit: allow branch/sha1/tag to be specified
workunit: allow branch/sha1/tag to be specified

Pull the workunit(s) from the branch/tag/sha1 specified in the config.

zhouyuan/teuthology,tchaikov/teuthology,michaelsevilla/teuthology,robbat2/teuthology,ceph/teuthology,SUSE/teuthology,zhouyuan/teuthology,SUSE/teuthology,michaelsevilla/teuthology,robbat2/teuthology,SUSE/teuthology,t-miyamae/teuthology,ceph/teuthology,dreamhost/teuthology,caibo2014/teuthology,dmick/teuthology,caibo2014/teuthology,ivotron/teuthology,t-miyamae/teuthology,ktdreyer/teuthology,yghannam/teuthology,ktdreyer/teuthology,ivotron/teuthology,yghannam/teuthology,dmick/teuthology,dmick/teuthology,dreamhost/teuthology,tchaikov/teuthology