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
Improve savanna-dashboard installation

* split configurations setting to one-per-line;
* don't set SAVANNA_URL in horizon configs - we're now using endpoits
  keystone to find corresponding edpoint.

Change-Id: I9497a511656a2f70e923b651c66c5ef2917a0939
Merge "Improve savanna-dashboard installation"
Merge "add heat to the default devstack service list"
Merge branch 'master' of github.com:openstack-dev/devstack