Add: Policy to ensure there is recoverable space for PG repairs

Ref: https://dev.cfengine.com/issues/6665

Note: This policy uses namespace. namespace was available in 3.5, but this is
the first time namespace has been introduced to the masterfiles policy
framework. It would cause issues for clients older than 3.5. I do not have to
use namespace, so let me know if you think I should change it.