use yumrepo to create repofiles under RHEL/Centos

[benibr]

If Exec with wget is used to create the repo files, it conflicts with the purge parameter of yumrepo resource which removes unmanaged repos.

Example:

 resources { 'yumrepo':
    purge =>   true,
 }
class { 'wireguard':
}

This would create the repo file in the first run, then empty it in the 2nd run and then never touch it again.
Therefore wireguard cannot be installed.

This could be fixed by using yumrepo to manage the repo under all systems which use yum.