Cleanup and make HACKING.rst DRYer

Reference the OpenStack hacking guide in HACKING.rst and remove
duplicate entries.  Add placeholder section for heat specific
rules.  heat specific rules can be created using hacking's local check
support.

Change-Id: Ib6967ae769bd73857abb7ef89368c407c8b22053
Merge "Cleanup and make HACKING.rst DRYer"