Only add ceilometer user to nova group if using compute agent

When using ceilometer outside of the compute context (ex: with the
swift middleware), puppet-ceilometer attempts to add the user to
the nova group which does not (and probably should not) exist on
the swift nodes.

Change-Id: I6cc68152325a967c024ce611f92a90ce0afba8cc
(cherry picked from commit d064997cd4c50d9b5365fe2ffca60e14a86b08e1)