The newly introduced YAML Loader plugin is currently hosted on the
same plugin (directory and module) as the varianter one.  While they
share a lot of code, they have different purposes.

Let's move the YAML loader into its own plugin, so that its
installation is also optional and the separation is clearer.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

Use the new ability to report full mappings after the discovery and
allow using `test_reference_resolver_class` to specify the resolver
class intended for test discovery.

To allow sufficient flexibility also support
`test_reference_resolver_args` to override resolver arguments and
`test_reference_resolver_extra` to extend the resolver extra_params.

Note the overridden arguments apply only to the resolver, not to whole
Avocado so it's not possible to override wrappers or such just for a
single test execution. Anyway it is possible to set `vt_config` just for
that single loader, which is more-than useful.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

Let's add yaml_testsuite loader which allows parsing YAML file and
producing testsuite out of it.

At this point due to Avocado limitation it only supports
FileLoader-based test discovery, but it should be possible with greater
changes to extend the scope of this loader for just about any Avocado
loader.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

Signed-off-by: NCleber Rosa <crosa@redhat.com>

Signed-off-by: NCleber Rosa <crosa@redhat.com>

New optional plugins are now available, and some in RPM package
format.  Let's include them where plugin RPM packages are listed.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

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>