gluon-core: add unique node_id

This adds a `node_id` to announce.d. `node_id` is an hex-number uniquely
identifying a particular node. On nodes having a
primary MAC this MAC address will be used as `node_id` (sans the
colons). The value is opaque. It must not be converted back into
a MAC address.

`node_id` should be sent in all alfred and announced packets to
ensure clients can link the data to a particular node.

gluon/gluon-announce/files/lib/gluon/announce/announce.d/node_id

@@ -0,0 +1 @@
+return require('gluon.sysconfig').primary_mac:gsub(':', '')