neutron: don't set external_network_bridge option by default

neutron: don't set external_network_bridge option by default

It's deprecated, to be removed in Ocata, and it's discouraged to set it
to anything but the default value ('') that means that routers are not
plugged directly into br-ex, but allows l2 agent to do the wiring.

There are known issues with setting it to br-ex (like wrong port
statuses): If533cf7c4c379be78f5a15073accaff7f65973ab

The only caveat to setting it to the default ('') value is that in that
case l2 agent should be configured with bridge mapping for physical
networks. Since we already configure bridge_mappings for the agent, we
should be safe to unset the option.

Now that it's the default, there is no reason to override it in example
environments.

This patch also changes the description for the parameter to make it
more clear that users are not expected to set it unless they know what
they are doing. Also, moved the parameter into deprecated section to
make it even more clear it's not something to touch in new deployments.

Change-Id: Iade7fbaf92c8c601227f4456a15ea3f13a907ee2
Related-Bug: #1563070