avocado.multiplexer: Add support for including files and removing node/values [v0]

Hi guys,

this pull request is built on top of the https://github.com/avocado-framework/avocado/pull/315 pull request, so please pay attention only to the part after  16030f7 `avocado.multiplexer: Support for debug run`. Currently I didn't pay attention to documentation as the concept might vary during review phase. I'll add it into the final version.

New features:

* Support for `!include` to include other yaml file (either absolute path, or relative to the __current__ yaml file.
* Support for `!using` to prepend multiple nodes (prepend only, It's impossible to move it outside the current position)
* Support for `!remove_node` to remove node (It's similar to `only` filter, but you're able to recreate any complex structure and reuse it. This feature should help building upstream->downstream configs.
* Support for `!remove_value` to remove value (The same usecase as `!remove_node`. This one I'm not fully convinced about as you can just change the value instead. I added it to double the `node` version).

I'd like to ask a question, do you think it'd be useful to have way to remove all nodes and/or all values from given node? And if so, would you like to have different tag for it, or just use `!remove_node : True/None` (it'd result into boolean, nodes can't be named this way so it's guaranteed not to collide with a name.

Regards,
Lukáš