avocado.plugins.multiplexer: Implement -tree option

Introduce the --tree option to the multiplexer plugin,
which shows the tree structure of the multiplex file.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.multiplexer: Remove duplicate method

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>