Cleanup and make HACKING.rst DRYer
Cleanup and make HACKING.rst DRYer

Reference the OpenStack hacking guide in HACKING.rst and remove
duplicate entries. Adds placeholder section for neutron specific
rules.

Change-Id: I4bfbab50b77e7592178dda44c7f4f52edc7fdc21

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