docs: ensure third party tests pass before +2
docs: ensure third party tests pass before +2

Adds a note to the code review guidelines to make sure appropriate third
party tests are passing before giving a change a +2 vote.

Change-Id: I42501d3996a2751b69803e37451cc0544ad75ce8

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