Fix up the 'disk' api

A recent patch to introduce support for creating disks from
snapshots had a few problems:
1. Incorrect URL (to the snapshot resource) was being sent
2. Ruby syntax errors.

I looked into adding Shindo tests for this, but am not able to
since this request relies on a pre-existing snapshot which we
use for disk creation.
Merge pull request #2068 from maginatics/gce_fix_disk_from_snapshot

[google|compute] Fix up the 'disk' api