avocado.multiplexer: Support for debug run [v3]

Hi guys,

there are bunch of improvements and support for debugging of the multiplex files. See the commit messages for details.

v1: #303
v2: https://github.com/avocado-framework/avocado/pull/313

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)
    v3: additional documentation for "TreeNode.__eq__"
    v3: "GREY" color is named "LOWLIGHT" now
    v3: Fixed typos in "tree.py"
    v3: Local imports in "tree_debug.py" were moved into global
    v3: Improved "TreeNode.set_environment_dirty" docstring