Deduplicate HACKING.rst with docs.openstack.org/developer/hacking/

Reference the OpenStack hacking guide in HACKING.rst and remove
duplicate entries. Add empty section as a place holder for barbican
specific rules.

Change-Id: Id1aecd08640fcca324bdb2028d6b3b6867a8b892
Merge "Deduplicate HACKING.rst with docs.openstack.org/developer/hacking/"