Set the volume access mode during volume attach

cinderclient supports an optional keyword argument named 'mode'
for volumes.attach(), which allows you to set the access mode of
a volume at attach time. The keyword argument is not passed in
by nova when it attaches a read-only volume, making the volume
access mode default to 'rw' instead of the desired 'ro'. This
patch finds the access mode of the volume and passes it to
cinderclient, so the correct access mode is used at attach
time.

Change-Id: Ib62f454292e60114f96debbad96c4a356dc32a21
Closes-Bug: #1307088
Merge "Set the volume access mode during volume attach"