Add a scatter-gather for multiregion job plans. Each region's servers
interpolate the plan locally in `Job.Plan` but don't distribute the plan as
done in `Job.Run`.
Note that it's not possible to return a usable modify index from a multiregion
plan for use with `-check-index`. Even if we were to force the modify index to
be the same at the start of `Job.Run` the index immediately drifts during each
region's deployments, depending on events local to each region. So we omit
this section of a multiregion plan.
Integration points for multiregion jobs to be registered in the enterprise
version of Nomad:
* hook in `Job.Register` for enterprise to send job to peer regions
* remove monitoring from `nomad job run` and `nomad job stop` for multiregion jobs
