BZ #1118067 - Intermittent incompletion of puppet run on compute nodes.

https://bugzilla.redhat.com/show_bug.cgi?id=1118067

It seems that ceilometer::compute depends on a nova user group existing, which
may or may not have been created yet with the existing code.  Add an ordering
dependency to make sure nova is installed and configured first.  Thanks @jayg
for helping debug.