Tweak collectd monitoring so that disk free reports actually work

[SJrX]

Is your feature request related to a problem? Please describe.
A year ago I opened #176 , but I never got around to doing it. Essentially we run out of disk space and that can be suboptimal. Collectd can report disk space metrics, but the config is invalid.

In #176 I had a complex solution, but there is a trivial one where we can just tweak the config.

Describe the solution you'd like
Report disk metrics for /

Describe alternatives you've considered
A complex system of env vars, also supplying additional configuration files.

Additional context
N/A

The MR might break anyone who actually has a /dev/vda device, but I think that's few and far between when running in docker.

[SJrX]

The MR is a low risk change, but I didn't actually test it because it's blocked by #205 .

[deniszh]

Merged both fixes for #205 and #207