workunit: allow setting environment variables
workunit: allow setting environment variables

This is useful for e.g. running the same tests against rbd in new and
old formats.

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