qa/workunits/rbd: tweaks to use RBD_MIRROR_USE_EXISTING_CLUSTER locally

qa/workunits/rbd: tweaks to use RBD_MIRROR_USE_EXISTING_CLUSTER locally

This var is mostly used when running rbd_mirror test scripts on
teuthology.  It can be used locally though to speedup re-running the
tests:

Set a test temp directory:

  export RBD_MIRROR_TEMDIR=/tmp/tmp.rbd_mirror

Run the tests the first time with NOCLEANUP flag (the cluster and
daemons are not stopped on finish):

  RBD_MIRROR_NOCLEANUP=1 ../qa/workunits/rbd/rbd_mirror.sh

Now, to re-run the test without restarting the cluster, run cleanup
with USE_EXISTING_CLUSTER flag:

  RBD_MIRROR_USE_EXISTING_CLUSTER=1 \
    ../qa/workunits/rbd/rbd_mirror_ha.sh cleanup

and then run the tests:

  RBD_MIRROR_USE_EXISTING_CLUSTER=1
    ../qa/workunits/rbd/rbd_mirror_ha.sh

Signed-off-by: Mykola Golub <mgolub@mirantis.com>