Single anax config file that is reread on registration

[bmpotter]

The anax config file situation should be cleaned up. There are currently 2 different config files, and they aren't both read (or reread) at all of the right times. There are several improvements that are all related:

• [ ] Use a config file framework that handles multiple config files in precedent order and produces consolidated internal map that honors that precedence. Use this capability to bundle a complete anax.json file that contains default values for every config parameter. This then enables the user to specify a minimal /etc/horizon/anax.json that contains only the config parameters he/she wants to override (like exchange url, etc.)
• [ ] With the bullet above supported, start phasing out /etc/default/horizon so there is only 1 kind of config file for anax.
• [ ] Have the agent reread /etc/horizon/anax.json on registration, so changes to mgmt hub values like exchange url or the cert are picked up.