Attach support for Flexvolume driver

[yasker]

We may need to support attach/detach in addition to current mount/umount. When Kubernetes think one pod is down on one node, it will create the same pod on another node. In the process, Kubernetes will try to detach the PVC from volume if possible, then attach to the new node and use it for the restarted pod.

Flexvolume document regarding attach support https://github.com/kubernetes/community/blob/master/contributors/devel/flexvolume.md#driver-invocation-model

But it also mentioned that
This call-out (attach) does not pass "secrets" specified in Flexvolume spec. If your driver requires secrets, do not implement this call-out and instead use "mount" call-out and implement attach and mount in that call-out.

Need to check if it's worth doing it, or simply do detach inside mount call is enough.

[yasker]

Skip this. Replace it with #50

We're switching to CSI for v1.10+.

1. It has been marked as beta in v1.10.
2. The deployment directory is not detectible for flexvolume after v1.9.