Add firewall with intra-cluster rules
Move zk cluster configuration to helper file

All you need to do is enable zookeeper and give unique ids.

Then include zk-cluster.nix in the network specification for nixops and
it will automagically generate the correct configuration for zk and
firewalll.
Hardcoded mesos master and slave
Auto-cluster mesos nodes
remove (harmless) empty zk specifications
Parametrize mesos cluster

- move zookeeper servers to masters
- add bash completion package
Install spark only on slaves
Calculate mesos quorum
Allow rollbacks
Limitations