add heat to the default devstack service list

heat has been integrated for a while, we should turn it on out of
the box. Also refactor the service list to make it simpler to
understand what's enabled.

Change-Id: I9738f39ce196d5c7f75b0a5b164222ea165fb340
Merge "add heat to the default devstack service list"
Merge branch 'master' of https://github.com/openstack-dev/devstack