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.

weave

@@ -443,10 +443,8 @@ EOF
 expose_ip() {
     ipam_cidrs allocate_no_check_alive weave:expose $CIDR_ARGS
     for CIDR in $ALL_CIDRS ; do
-        if ! ip addr show dev $BRIDGE | grep -qF $CIDR ; then
-            ip addr add dev $BRIDGE $CIDR
-            arp_update $BRIDGE $CIDR || true
-        fi
+        call_weave "POST" "/expose/$CIDR"
+
         [ -z "$FQDN" ] || when_weave_running put_dns_fqdn_no_check_alive weave:expose $FQDN $CIDR
     done
 }
@@ -1484,7 +1482,6 @@ case "$COMMAND" in
         fi
         create_bridge
         expose_ip
-        util_op expose-nat $ALL_CIDRS
         show_addrs $ALL_CIDRS
         ;;
     hide)