set http listen address solely through the weave script

Previously it was set by the script only in fastdp mode; and in the
router's main.go otherwise. Yet every interaction of the script with
weave assumes the port is the script's HTTP_PORT=6784. By always
starting the router with options to listen on that port we ensure that
the router is in fact listening where the script expects it to.

In doing this get rid of router.HTTPPort. We do need to hard-code the
port in the proxy instead, but relying on router.HTTPPort was flawed
anyway, for the above reasons.

proxy/proxy.go

@@ -17,7 +17,6 @@ import (
 	. "github.com/weaveworks/weave/common"
 	weavedocker "github.com/weaveworks/weave/common/docker"
 	"github.com/weaveworks/weave/nameserver"
-	"github.com/weaveworks/weave/router"
 )
 
 const (
@@ -490,7 +489,7 @@ func (proxy *Proxy) getDNSDomain() (domain string) {
 		weaveIP = "127.0.0.1"
 	}
 
-	url := fmt.Sprintf("http://%s:%d/domain", weaveIP, router.HTTPPort)
+	url := fmt.Sprintf("http://%s:6784/domain", weaveIP)
 	resp, err := http.Get(url)
 	if err != nil || resp.StatusCode != http.StatusOK {
 		return