Added need info to accept_transfer

Drivers that implement accept_transfer will need
things like the new user_id and project_id.  We were
not including this in the original add, in order for
drivers that are tenant aware this information will be
necessary.

Also the get_volume call in the transfer was using
the new user context, so the volume would never be
found.  We fix this here by providing an elevated
context to the get_volume call when accepting the
transfer.

Change-Id: I7b60c19950f85c4309a97bb842ff238bcf8e746a
Fixes: bug 1213275
Merge "Added need info to accept_transfer"