Refactor service user/group management

Refactor service user/group management

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