Make readme reference git.openstack.org not github
Make readme reference git.openstack.org not github

Point to our offical mirror instead of github.

Change-Id: I43c3d5ce4f92f35ed4fe5bddce5a5d19016ab988

dougwig/x-neutron-fwaas,citrix-openstack-build/neutron-fwaas,openstack/neutron-fwaas,gaolichuang/neutron-fwaas,openstack/neutron-fwaas,gaolichuang/neutron-fwaas