remove references and uses of fixed_range wrt nova.conf

Fixes: bug #1269876

As per commit https://github.com/openstack/nova/commit/1f6c4f5,
fixed_range in nova.conf is deprecated.  We don't need to be setting it
any more.

Change-Id: Ie9371a54c3bcbad3c45b851b67f78c38c43940bf