swift: raise container availability thresholds

it has been observed that while object availability changes when rebalancing,
container availability acts as a good proxy for e.g. a whole machine that's in
trouble.

if these thresholds don't yield false alarms the idea is to turn them into
critical because it might indicate a real problem with swift.

Change-Id: Id39d69344d54d3e01b86bcf580c6cda65a85ca3c