Don't use context.elevated to get volume

Don't use context.elevated to get volume

Original Problem:
=================
Since the metadata(readonly and attached_mode) is stored in admin metadata,
normal user need `run context.elevated` to retrieves admin metadata.
The above way will bring a side effect. Normal user can also get
any volume which the user shouldn't access when the user knows the UUID.

Solution:
=========
Use context instead of context.elevated to get volume.
And add admin metadata to it.
Based on cinder-meetup-summer-2015 conclusion, we use the solution.
The solution will need extra database connection.

Change-Id: I06f21e7578b65a59c0fe4d3afe0e882ed73c4725
Closes-Bug: #1477625