"cache_store" config value is not validated

[rhafer]

The value for setting the various gomicro stores we're using throughout reva is validated nowhere. If you set it to some unsupported values or have a type (e.g. I set nats-kv-js instead of nats-js-kv) it more or less silently (it tries log a error) falls back to the "memory" implementation. Which can have interesting effects e.g. for the posix storage driver.

We should have some form of validation. And maybe make store.Create() fail when called with an unknown store type.