qa/standalone: Fix test_activate_osd() test in ceph-helpers.sh

qa/standalone: Fix test_activate_osd() test in ceph-helpers.sh

Modify test_activate_osd() to get the type of scheduler in use and then
verify the value of osd_max_backfills. This is because mclock scheduler
overrides this option to 1000 upon OSD initialization.

The test earlier used to pass because the OSD daemon was killed but not
marked down and upon being brought up, the wait for OSD up check was
passing quickly. But the OSD still didn't have the latest config values.

But now upon killing the OSD, the osd_fast_shutdown sequence notifies the
mon (see PR: https://github.com/ceph/ceph/pull/44807) and is marked down
and dead. Upon bringing it up, the wait for OSD up check takes a longer
time and this is sufficient for the config values to be updated. This
results in the correct values being read from the config 'Values' map.

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