avocado.multiplexer: Support for debug run [v2]

Hi guys,

there are bunch of improvements and support for debugging of the multiplex files. See the commit messages for details.

v1: https://github.com/avocado-framework/avocado/pull/303

Changelog:

    v2: put debug-related stuff in 'avocado.core.tree_debug'
    v2: pass path value to yaml-parsing function
    v2: don't read yaml file, only pass the opened file (lower mem consumption)