Add collectd.

The collectd script is sourced by /etc/init.d/collectd when starting the
collectd service. This script resets the default CONFIG file to one
needed by M-Lab.
Add custom types.db for collectd-mlab resources.

The content of types.db for collectd is described in:
  https://collectd.org/documentation/manpages/types.db.5.shtml

The first field is the type name, and the second is identical to an RRD
'data source' specification. Data source (or DS) specs are described in
some detail here:
  http://oss.oetiker.ch/rrdtool/doc/rrdcreate.en.html
Add custom collectd configuration for M-Lab servers.

This adds the collectd-mlab.conf for M-lab servers. This is a
much-reduced version of the default /etc/collectd.conf with only the
plugins used by collectd-mlab. As well additional notes are included
that are particular to M-Lab's configuration.
Merge pull request #4 from stephen-soltesz/review-plugin-config

Add configuration files for m-lab's collectd plugin.