Add collectd to the monitoring suite

This change set builds `collectd` from source and configures it in one of the following ways:

- If Librato credentials are present, `collectd` will be configured to send data points to Librato using the `collectd-librato` plugin.

- If no Librato credentials are present, `collectd` will be configured to write RRD files locally (`/opt/collectd/var/lib/collectd/rrd by default`).

Aims to resolve https://github.com/al3x/sovereign/issues/253.