engine: Adding Speed optimization option to the scheduler

Add to cluster entity optimization_type field.
this field is a binary representation of cluster optimization possibilities.
The first option is optimize for speed, allows to skip host weighing in case
there are more than configurable amount of pending scheduling
requests.

Bug-Url: https://bugzilla.redhat.com/1014696
Change-Id: I0784f89648093e650be73fcc3d850a4854d53c1a
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>