Add disk space monitoring for snowflake infrastructure
We've run out of disk space at both the snowflake bridge (legacy/trac#26661 (moved), legacy/trac#28390 (moved)) and the broker (legacy/trac#29861 (moved)), which has caused snowflake to stop working. We've set up rotating and compressed logs but it would be nice to have some disk space monitoring to alert us if/when this happens again
Also, as discussed on IRC, we should eventually move the broker to a TPA machine.