Deprecate external_network_bridge option in L3 agent

Deprecate external_network_bridge option in L3 agent

This option provides another way to attach to a specific bridge
that is not quite equivalent with how bridge_mappings work in the
L2 agent. This creates inconsistencies between how the L3 agent
behaves when configured with a bridge_mapping and provider properties
of the Neutron network vs. when it just ignores all L2 stuff and
plugs itself directly into the bridge.

See the bug report for more info.

Change-Id: I37de3cd6eaaf34856fa72753f471f4f0a9381836
Closes-Bug: #1491668