Issue 502: Pravega Operator pre-delete hook

[SrishT]

Change log description

Provides a pre-delete hook which ensures that custom resources are deleted before deleting the operator.

Purpose of the change

Fixes #502

What the code does

Allows the operator to be deleted via helm only if the PravegaCluster custom resource has already been deleted, not otherwise.

How to verify it

• If you try to delete the pravega operator before having deleted the PravegaCluster custom resource, the pre-delete job will fail, thereby not allowing the operator from being deleted.
• If you try to delete the pravega operator after the PravegaCluster custom resource has been cleaned up successfully, the operator will also be successfully deleted.

[anishakj]

LGTM