Make every lqa command an object inheriting from a base Command
class.

This introduces a new hierarchy for commands where each of
them will live in its own file inside the commands module
(commands/submit_job, commands/cancel_job, and so on...).

This patch also includes the 'lqa_api' module which is intended
to offer convenient objects and methods to access LAVA api, and
it already adds a Job object that can be used to easily query
job information.