mgr/dashboard: Fix for some dashboard timing issues

mgr/dashboard: Fix for some dashboard timing issues

Specifically fixes the recurringly occurring `test_osd.py` error on the
`test_scrub` method. But this change should also prevent other issues of
the same kind. Issues of "same kind" are issues which occurr due to
tests which do not immediately result in a clean cluster status and
aren't manually programmed to wait for it.

Fixes: http://tracker.ceph.com/issues/36107
Signed-off-by: Patrick Nawracay <pnawracay@suse.com>