Create s4 status command

We should have an "s4 status" command that provides nicely formatted information about the state of S4 application coordinated through a given Zookeeper ensemble.

In particular:

• which applications are deployed
• on which S4 nodes these applications are deployed
• how many S4 nodes are running, how many S4 nodes are in standby
• which streams are published, which applications are connected

The implementation would probably add a new class in the s4-tools project.