Management framework: additional logging tweaks

Ensure the framework's log stream exists prior to using it in zeek_init(), and
use a node-is-live message similar to those in agent and controller also in
launched nodes.

scripts/policy/frameworks/management/node/main.zeek

@@ -118,4 +118,6 @@ event zeek_init()
 
 	Broker::peer(epi$network$address, epi$network$bound_port, Management::connect_retry);
 	Broker::subscribe(node_topic);
+
+	Management::Log::info(fmt("node %s is live, Broker ID %s", Cluster::node, Broker::node_id()));
 	}