avocado.multiplexer: Support for multiple yaml files [v2]

Hi guys, this pull request implements similar behavior as #271 only on top of the direct yaml parsing.

First couple of commits are just bugfixes and optimization. The unittests are adjusted as last commit and it covers 98% of the multiplexer and tree.

Regards,
Lukáš

v1: https://github.com/avocado-framework/avocado/pull/295

changelog:
    v1: Raise proper exception when yaml file doesn't exist
    v1: Multiple yaml files documentation + manpages update
    v1: Removed slower any_sibling implementation
    v1: Commit typo fix (Remove unnecessary branch...)
    v1: Added tree-merge unittest
    v1: Added multiple yaml-files functional test
    v1: Added commit "avocado.core.tree: Stop using OrderedDict for values"