Make DOCKERHOST_IP and DOCKERHOST_HOSTNAME a configuration option

[craffit]

Currently DOCKERHOST_IP and DOCKERHOST_NAME are not configuration options. This not work well when turning connectionbeat into a linux service, because linux services have their environment cleared. The configuration file would be a more logical place for these settings.

[raboof]

Passing those in through environment variables does make sense when connbeat is started in a docker container (12-factor and all that), but indeed when started from systemd it'd make sense to allow configuration from the configuration file. Let's see if we can support both.