Include python-file using the real path

When layout_config points to a symbolic link, Zuul attempted to load the
python function from directory of the symbolic link instead of the
parent of the link target.  That causes the scheduled to halt.

* Use realpath on layout_config path to make sure we will load from the
  proper place.
* Update documentation of python-file to mention the include is relative
  to the directory holding layout_config.

Change-Id: I33e221dd7c323423dbf781b53c06333dab2c7d29
Merge "Include python-file using the real path"