Only modify autoindex.rst if it exists
Only modify autoindex.rst if it exists

This fixes doc build errors when using older Sphinx versions.

Change-Id: I0ed9bed311d6309257c0a111671d0fea46f5da7c
Closes-bug: #1387343

openstack/oslo.concurrency,JioCloud/oslo.concurrency,varunarya10/oslo.concurrency