Fix floating ips with external gateway

If dnsmasq is configured to use an external gateway, then floating
ips on other interfaces do not work properly. This is because
outgoing traffic is no longer snatted to the floating ip.

This patch fixes it by adding an ebtables rule to force traffic
from ips that have a floating ip associated to route instead
of bridge.

Fixes bug 1096985

Change-Id: I8e4904660d42fe51c44b66686bed9f5d622693bd
Merge "Fix floating ips with external gateway"