As a user I can delete a manifest by digest via docker v2 api

[pulpbot]

Author: @ipanova (ipanova@redhat.com)

Redmine Issue: 7970, https://pulp.plan.io/issues/7970

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Enable DELETE on manifest endpoint

DELETE 	/v2/<name>/manifests/<reference> 	Manifest 	Delete the manifest identified by name and reference. Note that a manifest can only be deleted by digest.