bundles/icinga2: introduce, install checks, install sources.list, create postgres database

bundles/icinga2/files/systemd_override.conf

@@ -0,0 +1,9 @@
+[Service]
+# Icinga's default for this is "mixed". It assumes that check commands
+# spawned by icinga will exit quickly.
+#
+# sshmon tells openssh to spawn a master process for each node. Those
+# won't quit by themselves for a long time (this is the point). In order
+# to avoid a long waiting period while shutting down icinga, just kill all
+# processes in the cgroup.
+KillMode=control-group