Adds Persistent Disk volume type.

Implements a generalized Persistent Disk resource with plug-able API and Mounter interfaces.

Currently only supports GCE API, but the Persistent Disk resource was made general enough to support additional APIs.




My understanding is that GCE uses a virtio-scsi controller for disks, which should default to the sdx123 naming scheme. Either way, the code here uses the alias symlinks GCE provides to us in the dev-tree. The symlinks are in the format /dev/disk/by-id/google-mydiskname(-partX) and point to the actual device. If we can safely assume that naming scheme will point to the correct device name, we could probably assume this is an int for GCE PDs.