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
That's basically it, logging and error checking as the test config can be modified by the user (see ```available-services``` https://github.com/cobracmder/onie/blob/bbf36eea83d3be43ba73c8717f562d482b70d408/contrib/autobuild/config/onie-tests.json#L8-14 ).