Add support for configuring DNF package manager for Mageia 6+

Mageia 6 introduces DNF as an alternative package manager.

In addition, Mageia 6 switches the PackageKit backend to the DNF one,
meaning that the urpmi configuration does not apply to tools like
Apper, GNOME Software, and Plasma Discover, which use PackageKit
as the package management backend.

Since DNF (being the successor to Yum) is configured the same way
as Yum is, let's reuse the Yum configuration setup for Fedora to
configure it for Mageia.

We do this by changing to declare an array PACKAGEMANAGERS and
iterate through them and execute the actions for each PACKAGEMANAGER.

This way, we can easily support future transitions and parallel
package manager configurations from distributions in the future.

Bug: 729888
Change-Id: I19a80ee1e150d8cfea53944a2030e6ea0ae1ce44
Reviewed-on: https://chromium-review.googlesource.com/572394


Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488332}