Expire old inventory data

We need an ability to expire old inventory data after a certain time period (eg if the server has not been seen in X many days, then we should remove it from inventory, just like how old metric data is automatically removed).

Otherwise we could end up in a situation where the disk is getting too full of old data which is no longer useful.

We should probably use the same TTL setting that we have for p8s

This will be important for running on OpenShift as the persistent volume are limited by size.

It will also be important when monitoring servers running on OpenShift due to the ephemeral nature of pods (eg my replica set autoscales between 5 and 10 pods a day, when each new pod starts it will get its own inventory entry, multiple this by a few hundred different replica sets and after a year our disk could be filled with mostly outdated information)