always arp_update new addresses in 'attach'

Previously we'd only do that when the container had no ethwe at the
start. Therefore adding a recycled IP address to an already attached
container would not send out an arp update for that IP, which would
break connectivity to that IP for containers which had previously
accessed that IP.

Fixes #1405.

test/170_ip_reuse_test.sh

@@ -0,0 +1,31 @@
+#! /bin/bash
+
+. ./config.sh
+
+C1=10.2.1.43
+C2=10.2.1.44
+C3=10.2.1.45
+
+start_suite "re-using IP addresses (arp cache updating)"
+
+weave_on $HOST1 launch
+
+check() {
+    assert_raises "exec_on $HOST1 c1 $PING $C2"
+}
+
+start_container $HOST1 $C1/24 --name=c1
+
+start_container $HOST1 $C2/24 --name=c2
+check
+
+docker_on $HOST1 stop c2
+start_container $HOST1 $C2/24 --name=c3
+check
+
+docker_on $HOST1 stop c3
+start_container $HOST1 $C3/24 --name=c4
+weave_on $HOST1 attach $C2/24 c4
+check
+
+end_suite