qa/workunits/rbd: add env vars to adapt rbd_mirror.sh to teuthology
With these set, this script can work in teuthology, where we want to
control the how daemons run and their log locations, valgrind
settings, etc. The workunit task always sets CEPH_ID, and allows
other env vars to be configured optionally.
The RBD_MIRROR_USE_EXISTING_CLUSTER option can also speed up local testing.