Cleanup HACKING.rst
Cleanup HACKING.rst

* Point to new location for hacking doc
(http://docs.openstack.org/developer/hacking/)
* Remove openstack-common section because already in main hacking doc

Change-Id: I7fbcfe13469a75fed08700ccbe07fefbee33db22

openstack/neutron-vpnaas,openstack/neutron-vpnaas,dougwig/x-neutron-vpnaas,citrix-openstack-build/neutron-vpnaas