Make DriverBlockDevice save() context arg optional

We do not need to pass it explicitly since we wrap
BlockDeviceMapping object, which hold their own context.

Also remove an ugly use of get_admin_context() to get around the
limitation that existed previously.

Change-Id: Ide4d2e4e3fd1b456dec9e71939b63e913849c6cf
Merge "Make DriverBlockDevice save() context arg optional"