Cleanup and make HACKING.rst DRYer

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

Change-Id: Ia74da70363e3fe602405a440c1d2ec75052e9193
Merge "Cleanup and make HACKING.rst DRYer"