collectd: Add DaemonSet for collectd

Since all the nodes in the cluster run Fedora CoreOS now, we can
deploy collectd as a container, managed by a DaemonSet.

Note that while _collectd_ has to run as _root_ in order to collect
a lot of metrics, it should not run with all privileges.  It does need
to run as a "super-privileged container" (`spc_t` SELinux domain), but
it does _not_ need most kernel capabilities.

collectd/collectd.d/prometheus.conf

@@ -0,0 +1,5 @@
+LoadPlugin write_prometheus
+
+<Plugin write_prometheus>
+    Port 9103
+</Plugin>