Recently we created separate `yaml2mux` plugin, which keeps using the
`--multiplex` argument. The connection between yaml2mux and multiplex is
not really obvious and the separation was meant to bring the order to
the avocado params system. This is another step which replaces the
`--multiplex|-m` with `--mux-yaml|-m` and makes it more obvious that
it's related to `yaml` files and it's also related to `multiplexation`.

You might notice that the old `-m` was kept. It's very convenient and
also allows coexisting of old and new versions (useful eg. in remote
executions).
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>