Advanced Services documentation

Adds the beginnings of documentation for the Firewall as a Service,
Loadbalancer as a Service, and VPN as a Service.

Part of blueprint developer-documentation

Change-Id: I5e46df2dc4f87f961490ee75791a7e825ea94e1e
Merge "Advanced Services documentation"
Remove device_exists in LinuxBridgeManager

LinuxBridgeManager is adding a device_exists method instead of
using the one in ip_lib as the rest of the codebase

Change-Id: Ibeb95be76a1e1b20c69841e708c2a2cbf1cbe05a
Closes-Bug: #1302463