Don't collect metrics from overlay filesystems

This is typically used to mount Docker containers but it generates too
many volatile metrics which aren't useful.

Change-Id: I00117895570515b2c8f9690542e83061309464c3

linux/meta/collectd.yml

@@ -36,6 +36,7 @@ local_plugin:
     - tmpfs
     - fusectl
     - cgroup
+    - overlay
   linux_storage_disk:
     plugin: disk
     template: linux/files/collectd_disk.conf