use /dev/urandom instead of /dev/random to avoid blocking

...since in some typical deployment scenarios, e.g. small, minimal
VMs, not enough entropy may be available.

Fixes #170.

weave

@@ -109,7 +109,7 @@ delete_iptables_rule() {
 create_bridge() {
     [ ! -d /sys/class/net/$BRIDGE ] && {
         ip link add name $BRIDGE type bridge
-        ip link set dev $BRIDGE address 7a:$(od -txC -An -N5 /dev/random | tr \  : | tail -c+2)
+        ip link set dev $BRIDGE address 7a:$(od -txC -An -N5 /dev/urandom | tr \  : | tail -c+2)
         # Attempting to set the bridge MTU to a high value directly
         # fails. Bridges take the lowest MTU of their interfaces. So
         # instead we create a temporary interface with the desired