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/networking-mlnx,stackforge/networking-mlnx,openstack/networking-mlnx,moshele/networking-mlnx,stackforge/networking-mlnx,moshele/networking-mlnx-master