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.




Some piece of the system needs to run as root.  mount() is a very restricted syscall.  We can factor out all of those pieces into separate binaries, but it doesn't make sense for Danny to be on the hook for that.