avocado.core.tree: Docstrings, pylint/pep8 fixes

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
avocado.multiplexer: Support for debug run

This complex patchset adds debug version of yaml parser. It stores
the origin of each value from environment. For list it stores
per-slice origin too.

Debug mode is NOT suitable for run-ner, only for multiplex-er. It
corrupts the values by adding the origin after the value.

The unittests were adjusted and enhanced. I added explanation into the
examples/mux-selftests.yaml describing what the weird names stands for
and what they test. I added couple of check which stress the old tests
and couple of them test the new features.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>