Remove :dependent => :nullify for policy_events.

policy_events will be handled by a purger much like ems_events.

https://bugzilla.redhat.com/show_bug.cgi?id=1167110
Add purger for policy_events.

PurgerMixin has been created as a set of common methods from all of the
other purgers.  The other purgers will be modified to reuse these
common methods in the future.  Initially this mixin has been created
with methods for purging by date only, but ultimately it should include
methods for purging "by remaining" as is found in the miq_report_result
and drift_state purgers.

https://bugzilla.redhat.com/show_bug.cgi?id=1167110
Added scheduler methods for purging policy_events.

https://bugzilla.redhat.com/show_bug.cgi?id=1167110
Merge pull request #1117 from Fryguy/fix_ci_delete_with_policy_events

Fix issue where deleting a Cluster or Host tries to delete all policy_events