qa: update scrub start code to use comma sep scrubopts

qa: update scrub start code to use comma sep scrubopts

The documentation specifies this in [1] and yet we were using (I
believe) an older syntax:

    ceph tell mds.foo:0 scrub start / recursive force

instead of

    ceph tell mds.foo:0 scrub start / recursive,force

Oddly the former works at least as recently as in [2]:

    2021-06-03T07:11:42.071 DEBUG:teuthology.orchestra.run.smithi025:> sudo adjust-ulimits ceph-coverage /home/ubuntu/cephtest/archive/coverage timeout 120 ceph --cluster ceph tell mds.1:0 scrub start / recursive force
    ...
    2021-06-03T07:11:42.268 INFO:teuthology.orchestra.run.smithi025.stdout:{
    2021-06-03T07:11:42.268 INFO:teuthology.orchestra.run.smithi025.stdout:    "return_code": 0,
    2021-06-03T07:11:42.268 INFO:teuthology.orchestra.run.smithi025.stdout:    "scrub_tag": "cf7a74b2-3eb2-4657-9274-ea504b1ebf8f",
    2021-06-03T07:11:42.269 INFO:teuthology.orchestra.run.smithi025.stdout:    "mode": "asynchronous"
    2021-06-03T07:11:42.269 INFO:teuthology.orchestra.run.smithi025.stdout:}

[1] https://docs.ceph.com/en/latest/cephfs/scrub/
[2] /ceph/teuthology-archive/pdonnell-2021-06-03_03:40:33-fs-wip-pdonnell-testing-20210603.020013-distro-basic-smithi/6148097/teuthology.log

Fixes: https://tracker.ceph.com/issues/51146
See-also: https://tracker.ceph.com/issues/51145
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>