workunits: set CEPH_CONF environment
workunits: set CEPH_CONF environment

This allows any ceph util we run (including the rados-api tests) find
the config and keyrings they need.

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