Automate Sample Config file Generation

The sample config file is missing some key options (for example, RabbitMQ options).

Maintaining the complete list by hand is cumbersome and error prone, hence we originally went for a manual "just the necessities" option. Clearly this isn't perfect :)

oslo-incubator includes some code[1] intended to extract config options and generate the sample config files, we should investigate if this can be used with Designate reliably, and if so, make it happen.

Some quirks I can think of:

* We re-use some common options for SQL connections[2][3][4]
* If we automate this, we should ideally gate on it too. i.e. if a config option is added/removed/changed, Jenkins should -1 the patch unless the config file has been regenerated.
* Some libraries (e.g. oslo.messaging, once it's released) might define options.. How do we handle these?

[1]: https://github.com/openstack/oslo-incubator/blob/12bcdb71ffbe9ee1688beed1f0ddb0c198822682/openstack/common/config/generator.py
[2]: https://github.com/stackforge/designate/blob/master/designate/sqlalchemy/session.py#L37-55
[3]: https://github.com/stackforge/designate/blob/master/designate/storage/impl_sqlalchemy/__init__.py#L35
[4]: https://github.com/stackforge/designate/blob/master/designate/backend/impl_powerdns/__init__.py#L41-45