qa/standalone: Modify ceph-helpers.sh tests for mclock scheduler.

qa/standalone: Modify ceph-helpers.sh tests for mclock scheduler.

List of changes:

1. Remove the enforcement to use osd_op_queue=wpq when an osd is brought
   up in the following functions:
   - run_osd()
   - run_osd_filestore() and
   - activate_osd()

2. New functions:
   - get_op_scheduler() - Get the current osd_op_queue for an osd.

3. Modified test cases:
   - test_run_osd() - Add check for osd_max_backfill count.
     The mclock scheduler overrides the count to 1000.

4. New test cases:
   - test_activate_osd_after_mark_down()
   - test_get_op_scheduler()

Signed-off-by: Sridhar Seshasayee <sseshasa@redhat.com>