multiplexer: Introduce tag attribute

Tag is the path from root to the leaf. With this we can
properly tag tests. We change the root name from '/root'
to simply 'root' here.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado: Fetch test tag from multiplexer

Look at the test params to figure out the test tag.

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