Autotest ONIE

This is beginning of the testing framework for ONIE.

At the moment, it provides just the groundwork:
* Parameter validation
* Dynamic module loading
* Dynamic test definitions
* Basic documentation
I really think you should keep the test _data_ and test _config_ separate. Whether we use 'isc-dhcp` server or `dnsmasq` is a configuration choice, it's not part of your test-case, right?

Keep the config in a separate python config file (which itself is a python file, with a `.config` extension). Or you could choose any format that you're comfortable with.