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
Install Guide: Edit on ch_basics

As suggested by Diane in
https://review.openstack.org/#/c/55954/1/doc/install-guide/

Change-Id: Ide8aa9d8d1c81c331929f658b1148b2d88a316f6
backport: havana
Merge "Edits on section_nova-network.xml"