labs: adds link to external network script
labs: adds link to external network script

This patch adds install-guides link to
config_external_network and edits section name.

Change-Id: I1daa7f6cfb15dfc5a68494020f9952c1e2d6c93f

openstack/training-labs,openstack/training-labs,dguitarbite/OpenStack-Labs,dguitarbite/OpenStack-Labs,dguitarbite/OpenStack-Labs,openstack/training-labs