Clean up docs around required plug-ins

[asteflova]

          If Satellite depends on the plugins and are always present then shouldn't we hide the installation too?

We should also drop all --enable-foreman-compute-* sections for Satellite because they're mandatory. There's also a list of Foreman & Hammer plugins we always install so we should also drop the explicit enablement of those.

Originally posted by @ekohl in #3112 (comment)

[ekohl]

FYI: I'm working on clear guidance on the policy for Satellite. Right now it's very inconsistent and I'd prefer to clarify that.