Admin commands for setting config etc

[purplefox]

When working on k8s and tuning PranaDB it's very painful and slow to have to change config and redeploy the cluster each time.

I would like to introduce a new CLI command "admin" which has various sub commands that allow us to do things like change config settings, restart servers, wipe storage etc, so we can quickly iterate during performance tuning and testing.

E.g.

admin restart all servers
admin restart server 0
admin restart all servers and wipe storage
admin set config "data-cache-size"=10000000 and restart

By default admin commands would be disabled so not usable at the CLI. They could be enabled in server config for test clusters.