Squashed changes:

* Advertise hostname in server.properties
* Opening kafka ports
* lint; close ports
* Fixed up tests.
  * Tests were failing due to some ambiguities in the version of zookeeper
    that we are deploying. Fixed them so that they work with my setup (juju
    2.0, installing zookeeper from the charms store), and have some checks
    that should make them work with other setups.
  * Also fixed a mistake, where the deploy test was calling self.unit, but
    had set self.kafka.
* Added ability to override host.name (bind address) for kafka
    * Got rid of try-catch in tests, and fixed defaults.
    * config.yaml should default to null, rather than 0.0.0.0
      (They both do the same thing, basically, but 'null' results in nothing
      being written to server.properties.)

Changed name to "network_interface". You may now pass an interface name
or a CIDR range, and juju will figure out the correct ip address to drop
into the server.properties config.

Added documentation on the network_interface config value to README,
added tests, and made the way that variables get passed to
configure_kafka more consistent.

Without -r flag, hook won't auto retry.

Cleaning up "loose ends" from previous PR.