The `CreateSnapshot` RPC expects a plugin ID to be set by the API, but
in the common case of the `nomad volume snapshot create` command, we
don't ask the user for the plugin ID because it's available from the
volume we're snapshotting.
Change the order of the RPC so that we get the volume first and then
use the volume's plugin ID for the plugin if the API didn't set the
value.
