Relax policy so owner can access encryption info

The admin only policy is too restrictive to allow Nova to access a
volume's encryption metadata using the owner's request context. Hence,
this commit relaxes the policy for the volume encryption metadata
API extension so the metadata is accessible to the volume's owner.

Implements: blueprint encrypt-cinder-volumes
Change-Id: Ia946850b79f7f717ab7528caf7cac2905e650917
SecurityImpact
Merge "Relax policy so owner can access encryption info"