engine: scheduling: allow placement overbooking in cluster level

Adding a cluster optimization to enable parallel VM scheduling
requests for cluster (skip lock), in case pending requests are
greater than configurable threshold.
By default this feature is hidden from the user (unless
setting config.SchedulerAllowOverBooking to true).

Bug-Url: https://bugzilla.redhat.com/1014697
Change-Id: If52abf9974ab71b1f8d52ca268dac8dc39b89dc8
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>