Honor volume:get policy

Honor volume:get policy

The fix for bug 1356368 hard-coded a policy check (same as
rule:admin_or_owner) for volume:get.  While in most cases this is
what people want, it'd be good we honor policy setting.

Note that before commit 0505bb268942534ad5d6ecd5e34a4d9b0e7f5c04,
DB query volume_get() actually acted as the policy checker for
volume:get, and it raised VolumeNotFound if context.project_id didn't
match volume['project_id'].  The check_policy() in volume:get didn't
get a chance to raise PolicyNotAuthorized exception.  So in this
change we keep the same behavor.

Change-Id: If43cec5cce977b9220296709b4e243b35b06ecd5
Related-bug: #1356368