[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?
(minor style thing) For consistency, please use single quotes around strings.