ctl-district: act on many nodes with one invocation

Enhancements to help with:
https://trello.com/c/HWlGpenH/135-5-add-all-nodes-to-district-quickly

* Now you can specify multiple nodes to act on at once for all commands that take a server_identity
* list-available shows all nodes available for districting
* add-node can automatically create the district first
* add-node --available can add all undistricted nodes