Improve how docs describe adding repo to SLES

[htriem]

This contains a more efficient way of adding the Icinga repos to a SLES system. For more information, check out ticket number 42655.

[julianbrost]

ref/IP/42655

[julianbrost]

Where does the variable $releasever come from?

[htriem]

Well, I found this while looking for the answer: https://doc.opensuse.org/projects/libzypp/HEAD/zypp-repovars.html

It seems like every SuSE-related version uses this variable to determine the version of the OS.

[lippserd]

Yes, if it would be quoted. Have you tried to provide credentials?

[lippserd]

@htriem PING

[htriem]

I've provided them, still throws some errors. I've decided on a different way of adding the repos more in line with the RHEL way.

[htriem]

After some testing, I've changed the method to use wget, which is provided with SLES and more in line with the way we add repos under RHEL. Please check if this is more statisfactory.

Also, I've edited the related PRs as well.

[julianbrost]

What's wrong with doing the exact suggestion that was made in the ticket, i.e. just removing the {username}:{password}@ placeholders from the .repo file altogether?

Then the existing instructions work as-is an zypper asks for the credentials as needed:

0de8cd4736f8:/ # zypper ar https://gist.githubusercontent.com/julianbrost/5f90e5df34ee83a1bfd94cf545be9a6b/raw/af28f08635a29cbd97cee21a61b61b9e0fc1496c/ICINGA-release.repo
Adding repository 'ICINGA (stable release for SLES 15.4)' ................[done]
Repository 'ICINGA (stable release for SLES 15.4)' successfully added

URI         : https://packages.icinga.com/subscription/sles/15.4/release/
Enabled     : Yes
GPG Check   : Yes
Autorefresh : No
Priority    : 99 (default priority)

Repository priorities in effect:                (See 'zypper lr -P' for details)
      99 (default priority) : 36 repositories
     100 (lowered priority) :  1 repository
0de8cd4736f8:/ # zypper ref
[...]
Authentication required for 'https://packages.icinga.com/subscription/sles/15.4/release/'
User Name: [...]
Password: [...]
Retrieving repository 'ICINGA (stable release for SLES 15.4)' metadata ...[done]
Building repository 'ICINGA (stable release for SLES 15.4)' cache ........[done]
All repositories have been refreshed.

[htriem]

The Username and Password section have been removed from the .repo file, so there is no need for this PR anymore.