Merge branch 'ldoktor-yaml_testsuite_loader2'

avocado/core/loader.py

avocado/core/mux.py

avocado/core/runner.py

avocado/core/varianter.py

docs/source/optional_plugins/varianter_yaml_to_mux.rst

examples/yaml_to_mux_loader/advanced.yaml

examples/yaml_to_mux_loader/basic.yaml

examples/yaml_to_mux_loader/loaders.yaml

...ter_yaml_to_mux/avocado_varianter_yaml_to_mux/__init__.py

selftests/functional/test_loader.py