Don't pass volume type with snapshot ID on Cloudstack

- On Cloudstack restoring a volume gets mad when you pass a volume type
  as well.
- Remove redundant check.
- Fix documentation for get_volume_type_href method.
Merge pull request #18 from rightscale-cookbooks/st_14_16_acu185861_no_volume_type_with_snapshot_id_cloudstack

Don't pass volume type with snapshot ID on Cloudstack