Add ability to disable ceilometer-collector without db

You are no longer required to include ceilometer::db when
disabling ceilometer-collector using the enabled parameter.

The relationships with any database resources are now only enforced
when the service is enabled.

Closes-bug: #1352958
Change-Id: I00a5e651184788acdd30de1908e2bf38566f78e1
(cherry picked from commit 8c9f4fcd53e4ef0fb7694de6dce6e001c7e8cf7d)