Make External DB support available in foreman-{deb,el}

[ekohl]

Currently the guides are hidden (https://github.com/theforeman/foreman-documentation/pull/981). However, it can be made to work. What needs to happen:

• Package names & file need to be generalized. Software Collections are only used on EL7. Note this is also needed for EL8 support.
• The -evr package is only needed on Katello
• Only the Foreman DB is needed. Candlepin and Pulpcore are Katello-only
• postgresql-setup initdb is Red Hat only. On Debian the default apt install already initializes a DB and starts it.

[ekohl]

I opened https://github.com/theforeman/foreman-documentation/pull/2396 but that needs some additional work. I'll pick it back up after https://github.com/theforeman/foreman-documentation/pull/3167 because that greatly simplifies the whole procedure.