Add external patterns to configuration guide
Add external patterns to configuration guide

Description of new EAs was added to configuration_guide.rst:
- `External Domain Name Pattern`
- `External Host Name Pattern`
These EAs applies to external networks only.

Change-Id: Ib0cfc6aa8ad58305288004a1fbc19c230cc1b478

openstack/networking-infoblox,openstack/networking-infoblox