Monitor disk space on mirror machines
Monitor disk space on mirror machines

Add Icinga checks for disk space on:

- all filesystems
- the filesystem mounted at `/srv/mirror-data`

...on our off-site mirrors. These disk checks follow the same pattern as
our other disk space checks, in that they have a general check for all
filesystems and a specific check for each mountpoint of interest.

I've also added `Icinga::Host` resources necessary for these checks to
work.

There's quite a lot of duplication here but I'd rather refactor
that separately as I'm on Second Line and otherwise might not get this
story finished in time before I rotate off.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet