Fix: controller: ensure newly joining node learns the node names of none-DCs

Commit 0ec714adc covers the case where a node is rejoining. But for the
case where a node is newly joining controld cpg group, hence its node
name is not known by us yet, peer_update_callback() will directly bail
out with the logic:

if (node->uname == NULL) {
    return;
}

, so that non-DC nodes don't get to send out a hello message.

This commit fixes it by moving the hello messaging logic prior to that.