Remove user-facing images from cluster.yaml

[RobertLucian]

Description

Move docker image paths from cluster.yaml to cortex.yaml. Thanks, @deliahu, and @vishalbollu for thinking about this.

Motivation

Since #900 allows setting images on a per-API basis, keeping the afferent fields in the cluster config doesn't make much sense, because, at this idea's core, images are supposed to be set on an API basis.

The other advantage of setting them in the API is that the user has more granular control over what API uses what resource. And regardless, if users leave the fields empty (in the API config), they will get the default paths anyway.

[deliahu]

@RobertLucian I think we may want to fold this into #900, what do you think?

[RobertLucian]

@deliahu Hmm, so you're saying this ticket should be closed and have the description added to #900? I agree - this ticket is tied to #900.

[deliahu]

Keeping it as a separate ticket is totally ok! I was just thinking #900 should close both 👍 (which it sounds like your were too)

[RobertLucian]

Oh yeah, definitely. Both of them are gonna be closed at the same time.