Remove repeated openstack-config line from yum version

Reported with doc comment:
http://docs.openstack.org/havana/install-guide/install/yum/content/nova-compute.html#comment-1088743139

Cherry-pick from https://review.openstack.org/#/c/52845/

Change-Id: Ib3329e1213754b22f40e8eae38e8ba1901713f84
Partial-bug: 1242249