Handle "weave expose" in weaver

The change makes "weave expose" to call the "/expose" HTTP
endpoint implemented in router which exposes the bridge.

Such architectural change required as we need to know whether NPC is
enabled when exposing the bridge and weaver knows this fact the best.

Unfortunately, this change makes impossible to "weave expose"
when weaver is not running.

test/130_expose_test.sh

@@ -33,13 +33,6 @@ check_container_connectivity() {
 
 start_suite "exposing weave network to host"
 
-# expose/hide with a CIDR work prior to launching weave
-run_on1   "! $PING $EXP"
-weave_on1 "expose  $EXP/24"
-run_on1   "  $PING $EXP"
-weave_on1 "hide    $EXP/24"
-run_on1   "! $PING $EXP"
-
 weave_on $HOST1 launch --ipalloc-range $UNIVERSE
 
 start_container $HOST1 $C1/24 --name=c1