arping not issued for dynamic attach, delaying container visibility

[rade]

Our attach code looks like this (in pseudo-code):

attach() {
    if the ethwe interface already exists then
       add IPs to ethwe
       return
    create ethwe
    add IPs to ethwe
    arping IPs
}

So we don't arping if the ethwe already exists.

[rade]

steps to reproduce

$ weave launch
$ weave run 10.2.0.1/24 --name=foo -ti ubuntu
113f50933ee30b3793a12e14af8f9f24add7c3b6d1cd2542fc7d424f2c24c2be
$ weave run 10.2.0.2/24 --name=bar -ti ubuntu
9a78b1f2c82edd2ebe2de464fe31effb2ef31d9ac281cf5815142b47799c606a
$ weave run 10.2.0.3/24 --name=baz -ti ubuntu
177cafc9ddb77ff96fea0edcdb2c541d9548207966a7c837370471607e5ed4c3
$ docker exec foo ping -c 1 10.2.0.3
PING 10.2.0.3 (10.2.0.3) 56(84) bytes of data.
64 bytes from 10.2.0.3 (10.2.0.3): icmp_seq=1 ttl=64 time=0.065 ms

--- 10.2.0.3 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.065/0.065/0.065/0.000 ms
$ docker rm -f baz
baz
$ weave attach 10.2.0.3/24 bar
10.2.0.3
$ docker exec foo ping -c 1 10.2.0.3
PING 10.2.0.3 (10.2.0.3) 56(84) bytes of data.

--- 10.2.0.3 ping statistics ---
1 packets transmitted, 0 received, 100% packet loss, time 0ms