Edits on section_nova-network.xml

As suggested by Diane in
https://review.openstack.org/#/c/55950/1/doc/install-guide/section_nova-network.xml

Change-Id: Id5dd614d638fd9bd1bb620423842f4f15fe0b5d5
backport: havana
(cherry picked from commit 20f995710f08ce4af50465da1457d0f8cf4b147b)
Merge "Edits on section_nova-network.xml" into stable/havana