[LIBCLOUD-596] Add SSD support to GCE Compute driver

Contributing support for SSD persistent disks to Google Compute Engine driver.  SSD was recently introduced [https://developers.google.com/compute/docs/disks#typeofdisks].

Changed default disk size to 10 GB in test fixtures "just because".

@wrigri - mind taking a look?
`disk_type` and `disk_auto_delete` are extension arguments so please prefix them with `ex_` (`ex_disk_type`, `ex_disk_auto_delete`).