Rework gather-rpm script to use `dnf download` on EL8

[NeilHanlon]

Eliminated some checks that should only be applicable to EL7 distributions, as well as sourcing the version number from /etc/os-release.

dnf-pluginscore` must be installed for the download command to work, and to slim down images, it's often not available, so in this case we install the package silently in the background.

[tianon]

Unfortunately, the biggest wrinkle here is that this script isn't actually run inside the environment of the image-under-test, especially because we want it to still work for cases like clefos where the system running this script can't actually run the tools from the image itself. :sweat_smile: :see_no_evil:

[NeilHanlon]

Ah, yes that makes sense :)

I shall hack^W invent a way to do this

[tianon]

Ended up with a slightly simpler (although less ideal) approach in #59 -- thanks for giving it a shot though! :+1: