test *_on helpers should send non-result output to &2, so that asserts work

Previously, the result output would include:
Docker on: <command>
result output text

etc...
But this would cause: assert "docker_on ..." to fail

test/config.sh

@@ -63,21 +63,21 @@ greenly() {
 run_on() {
     host=$1
     shift 1
-    greyly echo "Running on $host: $@"
+    greyly echo "Running on $host: $@" >&2
     remote $host $SSH $host $@
 }
 
 docker_on() {
     host=$1
     shift 1
-    greyly echo "Docker on $host: $@"
+    greyly echo "Docker on $host: $@" >&2
     docker -H tcp://$host:2375 $@
 }
 
 weave_on() {
     host=$1
     shift 1
-    greyly echo "Weave on $host: $@"
+    greyly echo "Weave on $host: $@" >&2
     DOCKER_HOST=tcp://$host:2375 $WEAVE $@
 }