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

Tehsmash/networking-cisco-nexus,Tehsmash/networking-cisco-apic