Fix backup instructions for non-Katello

[ekohl]

Not a blocker for this, but I don't think we have an instruction to install foreman-maintain anywhere.

Also, for the estimating the backup size the paths are a bit off. I have #1825 which would fix it for the PG part. Then we also have katello-specific paths there which should also be removed from foreman-el.

On a related note: I don't get the math in the example. How does it get to 681 GB?

The chapter 6.3. Performing a Backup without Pulp Content should be skipped on non-Katello.

6.6. Performing an Online Backup does talk about Pulp a lot. I also wonder how much of that is still true with Pulp 3.

6.7. Performing a Snapshot Backup also talks about Pulp a lot.

7.3. Backup and Restore Smart Proxy server Using a Virtual Machine Snapshot also has this:

If required, deploy a new Smart Proxy server, ensuring the host name is the same as before, and then install the Smart Proxy certificates. You may still have them on Foreman server, the package name ends in -certs.tar, alternately create new ones.

That is also Katello-only.

I don't consider the Katello-isms as a blocker, since the foreman-el guide is already showing the invalid instructions, but we shouldn't forget about it.

Originally posted by @ekohl in #1864 (review)

[ekohl]

#693 was already open, but I think this is easier to act on so keeping this open.

[apinnick]

@ekohl Please update.