Remove duplicate default=None for option compute_available_monitors
Remove duplicate default=None for option compute_available_monitors

In oslo.config, all type config options have default value None if
don't set parameter default. So don't need set config option
compute_available_monitors' default value with None in Nova.

Change-Id: I843e0ca4339ceefa54a7441ca245b8bc73d0139e

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