The flex volume wrapper interface changed significantly with Kubernetes
1.6, current interface described here:
https://github.com/kubernetes/community/blob/master/contributors/devel/flexvolume.md

Adapt code to the new set of methods. Namely:
* add structs for JSON marshaling of responses
* add constants for status values on responses
* add generic methods for flex volume responses
* change init to reply with the driver's capabilities
* add placeholders for unsupported commands (attach, detach,
  getvolumename, isattached, mountdevice, unmountdevice, waitforattach)

In addition add a prefix option for accessing /cvmfs when using kubelet
as a system container (/ gets mounted under /rootfs).