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
Fix ceilometer agent notification package error

ceilometer-notification package name should be
openstack-ceilometer-notification in RHEL.
See more info in related bug tracker record.

Change-Id: If64f1ea641c5d99c0adf2104a308c2d5442c3324
Closes-Bug: #1353844