multiplexer: Construct the tree directly from yaml file [v1]

Instead of creating OrderedDicts and then converting it into
tree structure, this modifies the `mapping` constructor to create
the tree structure directly. This way we only walk through the yaml
file once.

Also remove the `tree_unittest.TestReadYAML` selftest as it lost it's
meaning. Valures are checked in the TestTreeNode selftest.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>

v0: https://github.com/avocado-framework/avocado/pull/272

* v1: use CLoader when available
* v1: Raise error when adding 2 same children (they need to be properly merged, TBD)
* v1: Support for `str2TreeNode` comparism + selftest
* v1: Major style cleanup (of the new parts)
* v1: Removed `tree_unittest.TestReadYAML` (not applicable now)
* v1: Fix framework to use correct functions to interact with new functions.