Allow independent collectd

[madhadron]

Running collectd as a subprocess of the JVM makes it very easy to set things up, but we would like to be able to use an existing collectd running as a separate systemd service.

We propose adding two configuration fields:

• A flag to disable starting collectd
• A field to specify the path of the Unix domain socket to write to.

By default MCAC would continue to use its packaged collectd and a generated socket path. If these are specified, it would skip that and write to the specified socket if it exists.