This change refactors how the ceilometer service user and group are
managed.
- The ceilometer service user and group are created by the common
package. While the user resource should still be declared to manage
its group membership, we don't need the group resource.
- Introduces the configuration knob to disable user/group management.
This would be useful in the case where all service users are
declared externally.
Change-Id: Iaabe5b02f0ebd782debd0f3ca41e2fdafbf9c80f
2298f1ce55