Make persistence configurable at the Shard level

Currently we allow configuration of persistence only at the datastore level. In datastore.cfg you have the following configuration options.

persistent=true
operational.persistent=false
config.persistent=true

Some people may want operational inventory to also be persisted. It would be nice if they could configure it like so in the datastore.cfg.

operational.inventory.persistent=true

This would override the persistence option for the inventory operational shard.