qa/workunits/rbd: switch rbd-mirror workunits to bash

qa/workunits/rbd: switch rbd-mirror workunits to bash

By making use of here strings in commit ea3a567f7f03 ("qa/workunits:
make wait_for_status_in_pool_dir() reentrant") we grew a dependency on
bash.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 166a2362378b1ff93e43f483f354c428fd6cef9e)
Signed-off-by: Ramana Raja <rraja@redhat.com>
Conflicts:
	qa/workunits/rbd/rbd_mirror_journal.sh
        -  Commit 3fd8a03887354 not backported
           "qa/workunits/rbd: merge journal and snapshot test scripts"