qa/tasks/ceph: do not print out empty list of pg

qa/tasks/ceph: do not print out empty list of pg

we could have following logging messages:

tasks.ceph:Waiting for all PGs to be active+clean and split+merged, waiting on ['2.6', '2.5', '1.0', '2.4'] to go clean and/or [] to split/merge

if the cluster has non-active+clean pgs when the "ceph" is about to
end. but this message is a little bit confusing in the sense it
lists "[]" in it.

in this change, only PGs being waited are listed. also, added some
cleanups:

* use "else" to check if the loop is terminated by a break
* remove "0" from the range() call

Signed-off-by: Kefu Chai <kchai@redhat.com>