don't allocate IPs on 'detach' and 'hide'

Instead of POSTing to IPAM in order to obtain IPs, we GET existing
IPs. We ignore 404s, so detaching/hiding remains idempotent.

Fixes #860

test/510_detach_hide_no_ipalloc.sh

@@ -0,0 +1,26 @@
+#! /bin/bash
+
+UNIVERSE=10.2.3.0/24
+
+check() {
+    CMD="weave_on $HOST1 $@"
+    assert_raises "timeout 10 cat <( $CMD )"
+}
+
+. ./config.sh
+
+start_suite "'detach' and 'hide' do not require IP allocation"
+
+weave_on $HOST1 launch --ipalloc-range $UNIVERSE --init-peer-count=2
+
+start_container $HOST1 10.2.1.1/24 --name=c1
+check detach           10.2.1.1/24 c1
+check detach       net:10.2.3.0/24 c1
+check detach                       c1
+
+weave_on $HOST1 expose 10.2.1.2/24
+check hide             10.2.1.2/24
+check hide         net:10.2.3.0/24
+check hide
+
+end_suite