Adds a `-global` flag for stopping multiregion jobs in all regions at
once. Warn the user if they attempt to stop a multiregion job in a single
region.
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