Add option to control where bridges forward

This adds a new config option to control where bridges can forward.
The default behavior is unchanged, but if you specify:

forward_bridge_interface=foo

Then only traffic on the same bridge or going to and from the foo
interface will be allowed. This is useful in vlan mode when a
deployer doesn't want traffic to be routed between vlans. Note
that forward_bridge_interface can be specified multiple times and
should be specified once for each interface that supports floating
ips. i.e.:

forward_bridge_interface=eth0.10
forward_bridge_interface=eth0.20

DocImpact

Change-Id: I82d6b288b60cd6deae64690b55546730b1c2da55
Merge "Add option to control where bridges forward"