Clean up and make HACKING.rst DRYer

Reference the OpenStack hacking guide in HACKING.rst and remove
duplicated entries. Add new section for nova specific rules.

Change-Id: I704a49a0675acf1870953c76eba4978a1c4490eb
Merge "Clean up and make HACKING.rst DRYer"
Merge remote-tracking branch 'm_src_repo/citrix-fixes' into build-temp