avocado.plugins.yaml_to_mux: Use custom classes in tree creation

The `_create_from_yaml` function allows specifying custom tree node
object, but it only works properly with `tree.TreeNode` inherited ones.
Let's use the custom class instead to make the code cope with different
use cases.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/plugins/yaml_to_mux.py

@@ -66,7 +66,7 @@ def _create_from_yaml(path, cls_node=tree.TreeNode):
         node = cls_node(str(name))
         using = ''
         for value in values:
-            if isinstance(value, tree.TreeNode):
+            if isinstance(value, cls_node):
                 node.add_child(value)
             elif isinstance(value[0], tree.Control):
                 if value[0].code == YAML_INCLUDE: