Change a few default config values and improve application of common storage config

[trevorwhitney]

What this PR does / why we need it:

This PR represents a bunch of small changes that I uncovered when preparing my Obscon demo for the Simple Scalable Deployment architecture. One goal of this project has been simplifying configs so they are closer to how we run Loki.

• default ruler api to on
• register defaults for storage configs in the common config (ie.
  signature version for S3)
• enable ingester wal by default
• disable chunk retries by default
• allow common path prefix to include or omit trailing slash
• allow partial storage config to be defined for overriding things like
  bucket names per component

Checklist

• Documentation added
• Tests updated

[dannykopping]

Looks great! Will wait for another maintainer to review who's more familiar with the problem at hand

[owen-d]

I'm a little torn on whether we should make the WAL a default or not. On one hand, it's much safer to run with a WAL as it greatly alleviates durability concerns, but it also adds an explicit requirement on disk, which is a tricky dependency from an operator's perspective. There's also a secondary question: Will this break existing users upgrading to 2.4?

[trevorwhitney]

We can take the WAL bit out and maybe wait for a breaking release (3.0 anyone?), but I do think long term this is a change worth making. The advantage this brings re: durability is big, and in terms of breaking existing installations, I would be curious how common a practice it is to run loki without a WAL? I couldn't find any environment, dev or prod, at GL that was running loki in this way, which makes me think most production environments are running loki with a WAL.