The collector start the metering sample cleanup
process every 'database_clear_expired_data_interval' seconds to
remove sample according the 'database_time_to_live'
If the backend support ttl natively (mongodb >= 2.2), the cleanup
function in the backend is just a dump, and the backend handles the TTL
itself.
If the backend doesn't support ttl at all, it will raise a
NotImplementedException caught by collector to fill the log correctly.
Blueprint: db-ttl
Change-Id: I869ce6f50065d0ae8d7095a260efbfcd33165eef
2c2a07a688