Fail puppet-lint on warnings

Puppet-lint will now fail if there is any warnings.

Change-Id: I49e181bdfd0b27a61bf7242accd352a3625eaf93
(cherry picked from commit ffde8588fd1360335fbfd376e00f8d53b0ef05e2)
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)
Merge branch 'grizzly' of https://github.com/gwdg/puppet-ceilometer into grizzly