osd/scrub: increasing max_osd_scrubs to 3

osd/scrub: increasing max_osd_scrubs to 3

Bug reports seem to hint that the current default value of
'1' is too low: the cluster is susceptible to scrub scheduling
delays and issues stemming from local software/networking/hardware
problems, even if affecting a very small number of OSDs.

Squid will include a major overhaul of the way scrubs are counted
in the cluster, providing a better solution to the problem. For
now - modifying the default is an effective stop-gap measure.

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>