Clean up and make HACKING.rst point to openstack-dev/hacking

Instead of having a full local copy of HACKING, reference the OpenStack
hacking guide (openstack-dev/hacking) and remove duplicate sections.

Change-Id: Ief9ed232d03f196647870f7cbc818c9b2528d3d8
Merge "Clean up and make HACKING.rst point to openstack-dev/hacking"