Currently the yaml_to_mux plugin is a core plugin without properly
defined requirements. Let's separate it to optional_plugins, define
dependencies and also build it separately in RPM.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

The "avocado multiplex" command is misleading as multiplexation is only
the generic concept used by one of our varianter plugins
implementations. Let's change the command name to "avocado variants"
which better reflects what is it used for.

As people depend on "avocado multiplex" command let's keep both of them
for now to give people time till Avocado 52.0 to migrate.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

Some spelling issues, broken link fixes and updates related to recent
changes in Avocado.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

This adds the support to define per-node filters, which take part of
environment and are evaluated during multiplexation.

The basic idea is to define relations ({could be only used | could
not be used} with other branch), which makes it a bit different from
what we know from `--mux-filter-{only|out}`:

1. They are applied per complete variant (because the filters are
   compound of per-leaf-filters)
2. They don't remove/keep the nodes, but they only evaluate whether
   this resulting variant makes sense (whether it passes the filters)

See the updated documentation for details.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

All the plugin interfaces defined in `avocado.core.plugin_interfaces`
ommit the "plugin" term, simply because this is already qualified in
the module name.

Let's also standardize the Varianter interface to "plugin-less" name.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

Finally after all the refactoring, refining and modifying here is the
up-to-date documentation which follows the current structure and tries
to describe both theoretically and practically how are test's parameters
produced and how they can be used.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>