Fix references to examples in api documentation

The documentation displayed references to examples in the main docs but not in
the corresponding api reference docs.  The build had these warnings..

: WARNING: Include file u'/Users/khaido/workspace/jenkins-job-builder/doc/source/api/
../../tests/properties/fixtures/batch-task.yaml' not found or reading it failed

As pointed out by Darragh[1] the problem was that literalinclude needs a leading '/' to
correctly reference both docs.

[1] https://review.openstack.org/#/c/48783/15/jenkins_jobs/local_yaml.py

Change-Id: Ib1f8ccca40af1ee54c2bd00b2b42ba39e3c0c8e5
Merge "Fix references to examples in api documentation"