documentation: set sys.path to use autotest library.

Set sys.path in conf.py to use autotest library without exporting PYTHONPATH.
Required for proper builds of HTML on autotest.readthedocs.org site.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #769 from ruda/api-doc4

documentation: set sys.path to use autotest library.