Modify max_outgoing_migrations default value to 3

With default 5 and migration_max_bandwidth 32MiBps, over 1Gbps link the
host's link become saturated.

Change-Id: I445f3ce722583acfea384f057b8f5340303c2bf3
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=956683
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/14251
Reviewed-by: Michal Skrivanek <michal.skrivanek@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>