Add parallel test execution section to HACKING.rst

This commit adds a section to HACKING.rst to add some things to watch
out for now that we are running tempest in parallel by default. Part
of this documentation was taken from David Kranz's mailing list post:
http://lists.openstack.org/pipermail/openstack-dev/2013-August/014472.html

Change-Id: Ie688650fc528cf849c619e9afb64ba1f6b556ff8
1 file changed