The `--multiplex` is about to be changed into a separate plugin. It does
not sound fair to keep it as default positional argument of the
`multiplex` command. Let's sync the naming scheme and require
`--multiplex` for it.

This also means the `-m` is optional so one can run `avocado multiplex`
without arguments to get default multiplex values.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>