Rectify value of `AdvertiseAddrWan` when set elsewhere

`AdvertiseAddrs` has been introduced as a configuration option, which
duplicates a few other options, namely `AdvertiseAddrWan`. We need to
use this value elsewhere, so rather than doing a precedence check every
time we need to access it, rectify the value of `AdvertiseAddrWan` to
match

command/agent/agent.go

@@ -287,6 +287,7 @@ func (a *Agent) consulConfig() *consul.Config {
 	if a.config.AdvertiseAddrs.SerfWan != nil {
 		base.SerfWANConfig.MemberlistConfig.AdvertiseAddr = a.config.AdvertiseAddrs.SerfWan.IP.String()
 		base.SerfWANConfig.MemberlistConfig.AdvertisePort = a.config.AdvertiseAddrs.SerfWan.Port
+		a.config.AdvertiseAddrWan = a.config.AdvertiseAddrs.SerfWan.IP.String()
 	}
 	if a.config.AdvertiseAddrs.RPC != nil {
 		base.RPCAdvertise = a.config.AdvertiseAddrs.RPC