Fix image metadata returned for volumes

When creating a volume from a glance image, cinder stores the original
image metadata in volume_glance_metadata. This is a key/value store,
and all the values are strings.

When Nova boots an instance from a volume, it passes the image
metadata returned by cinder, which is all strings. If a driver
expects these values to be ints, as they are when booting from an
image, it will get a type error.

This change also pulls size from the volume directly rather than
taking the value from the stored image metadata. This is because the
volume will have been created in 1Gb increments, and is unlikely to be
the same size as the original image. It may also have been
subsequently extended.

Change-Id: I7928f6be1ca99f1502941b9df2b443f2ca63a37b
Closes-Bug: #1367540
Merge "Fix image metadata returned for volumes"