Remove mention of nova-manage.conf from nova-manage.rst

As of commit 07ee9639 three years ago nova-manage.conf is no longer
used, so let's clean up the man page.

Closes-Bug: #1304580

Change-Id: Ie061733da4bd0337f510e17b6dd7ca5bae14b7de
Merge "Remove mention of nova-manage.conf from nova-manage.rst"