The CSI specification allows only the `file-system` attachment mode to have
mount options. The `block-device` mode is left "intentionally empty, for now"
in the protocol. We should be validating against this problem, but our
documentation also had it backwards.
Also adds missing mount_options on group volume.
Includes:
* changes `nomad alloc status` and `nomad node status`
* changes to `volume` block in jobspec
* new `csi_plugin` block in jobspec
* new `nomad volume register/deregister/status` commands
* new `nomad plugin status` command