Change default policy for volume_extension:volume_tenant_attribute

The volume_tenant_attribute extension should be accessible by
the owning tenant and the admin. Change default policy from
'rule:admin_api' to 'rule:admin_or_owner'.

Change-Id: I886cd717741c86b5083aeaf16e3e7ea5b4e525b4
Closes-bug: #1269312
Merge "Change default policy for volume_extension:volume_tenant_attribute"