Edited multiple floating IP pools in Cloud Administrator Guide

1. Removed steps to configure multiple L3 agents for multiple external networks
2. Added configuration of a L3 agent to handle multiple external networks.

Change-Id: I7ff2f01d40a84a6c7ebd06a0b3fbdd527cd12c9d
backport: icehouse
Closes-Bug: #1380585
Merge "Edited multiple floating IP pools in Cloud Administrator Guide"