WIP: Introduce the new multiplexer.

Introduce the new multiplexer with treenode data type. Things touched:

* avocado.tests: Add yaml and remove mplx from the tests data.
* avocado.multiplex: the new multiplexer.
* avocado.job: the logic to create test variants.
* avocado.core.tree: Treenode data type.
* avocado.selftests: The functional and unit tests for the multiplexer and treenode.
* avocado.plugins.multiplexer: update the plugin to display
                               a visual tree (by lmr@redhat.com).

Signed-off-by: Ruda Moura <rmoura@redhat.com>