adds config validation, similar to cortex

[owen-d]

Background

After the 1.0 vendoring, we're exposed to some tricky configuration errors because we instantiate our configurations differently than cortex but use cortex's underlying configs. In this case, we loaded the schema's PeriodConfig blocks via yaml, causing subsequent calls to Load to no-op (this is intended to be an idempotent method).

What

This PR introduces a Config.Validate function and calls it in the loki execution path - it mainly calls cortex validation code similar to cortex: https://github.com/cortexproject/cortex/blob/master/cmd/cortex/main.go#L83-L89

Cortex Load: https://github.com/cortexproject/cortex/blob/master/pkg/chunk/schema_config.go#L228-L239

I've tested this fix on one of our clusters and confirmed we're no longer seeing panics due to 0 (default go value) shard factors in our configs.

[cyriltovena]

Lgtm