Avoid needless list copy in 'scheduler_host_subset_size' case
Avoid needless list copy in 'scheduler_host_subset_size' case

When 'scheduler_host_subset_size' is set to be greater or
equal to 'len(weighed_hosts)' there is really no need to create
a temporary list of the same size and then choose a random element
from that list.

Instead just select a random element from the original list, if
the size is greater or equal, no need to make a copy.

Change-Id: I1115f21d2bc5d77d13695e4e0708a9bd6915c5ec

vmturbo/nova,Juniper/nova,klmitch/nova,rahulunair/nova,rahulunair/nova,gooddata/openstack-nova,alaski/nova,cloudbase/nova,mahak/nova,gooddata/openstack-nova,gooddata/openstack-nova,mahak/nova,BeyondTheClouds/nova,cyx1231st/nova,rajalokan/nova,CEG-FYP-OpenStack/scheduler,rahulunair/nova,jianghuaw/nova,rajalokan/nova,openstack/nova,mikalstill/nova,klmitch/nova,jianghuaw/nova,cloudbase/nova,bigswitch/nova,vmturbo/nova,phenoxim/nova,jianghuaw/nova,cernops/nova,BeyondTheClouds/nova,NeCTAR-RC/nova,jianghuaw/nova,BeyondTheClouds/nova,bigswitch/nova,Juniper/nova,cyx1231st/nova,klmitch/nova,mikalstill/nova,openstack/nova,rajalokan/nova,hanlind/nova,sebrandon1/nova,mikalstill/nova,cloudbase/nova,hanlind/nova,cernops/nova,phenoxim/nova,gooddata/openstack-nova,klmitch/nova,dims/nova,zhimin711/nova,CEG-FYP-OpenStack/scheduler,alaski/nova,zhimin711/nova,hanlind/nova,vmturbo/nova,dims/nova,cernops/nova,openstack/nova,vmturbo/nova,Juniper/nova,sebrandon1/nova,mahak/nova,sebrandon1/nova,NeCTAR-RC/nova,Juniper/nova,rajalokan/nova