Consolidate all nomad data dir in a single root
`/tmp/nomad-dev-cluster`. Eases clean up.
Allow running script from any path - don't require devs to cd into
`dev/cluster` directory first.
Also, block while nomad processes are running and prapogate
SIGTERM/SIGINT to nomad processes to shutdown.
This removes an unnecessary shared lock between discovery and heartbeating
which was causing heartbeats to be missed upon retries when a single server
fails. Also made a drive by fix to call the periodic server shuffler goroutine.
