qa: choose victim pg from rbd pool

qa: choose victim pg from rbd pool

Right now scrub_test picks any pg in ceph. Unfortunately, it picked the
.mgr pool's only pg in [1]:

	2021-05-16T11:36:35.035 DEBUG:teuthology.orchestra.run.smithi049:> adjust-ulimits ceph-coverage /home/ubuntu/cephtest/archive/coverage rados --cluster ceph --pool rbd setomapval main.db-journal.0000000000000000 key val

Instead, only pick a pg in the rbd pool.

[1] /ceph/teuthology-archive/kchai-2021-05-16_11:19:39-rados-wip-kefu-testing-2021-05-16-1043-distro-basic-smithi/6117396/teuthology.log

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>