* Add feature flag to enable discovery and use of public IPaddr for clustering.
Before this change, Alertmanager would refuse to startup if using a
advertise address binding to any address (0.0.0.0), and the host only
had an interface with a public IP address. After this change we feature
flag permitting the use of a discovered public address for cluster
gossiping.
Signed-off-by: Devin Trejo <dtrejo@palantir.com>
fad796931b