This changeset corrects handling of the `ValidationVolumeCapabilities`
response:

* The CSI spec for the `ValidationVolumeCapabilities` requires that
  plugins only set the `Confirmed` field if they've validated all
  capabilities. The Nomad client improperly assumes that the lack of a
  `Confirmed` field should be treated as a failure. This breaks the
  Azure and Linode block storage plugins, which don't set this
  optional field.

* The CSI spec also requires that the orchestrator check the validation
  responses to guard against older versions of a plugin reporting
  "valid" for newer fields it doesn't understand.