shorten connection timeout for http interactions with containers

This issue came to light in the context of `weave launch`. When the
weaver container died after we've completed network configuration, it
would take a minute for the script to terminate, due to `curl` being
stuck in a connection attempt.

Fixes #538.

weave

@@ -429,7 +429,7 @@ http_call_ip() {
     http_verb="$3"
     url="$4"
     shift 4
-    curl -s -X $http_verb "$@" http://$ip:$port$url
+    curl --connect-timeout 3 -s -X $http_verb "$@" http://$ip:$port$url
 }
 
 # Perform operation $1 on container ID $2 to local DNS database at address $3