WIP: Add Scientific Linux distribution update plugin

This is my first stab at a distribution upgrade detection for Scientific Linux.

There is a fair bit of room for improvement here.... and I'm having a bit of trouble getting libxmlb enabled gnome-software to build on SL7.6.... so this isn't tested yet..

Basically this plugin downloads the repomd.xml and looks at the revision field[1]. Upgrades between major releases are not supported so the minor version is what matters. The release can go up to 5.11 so I'm splitting on the . so I can use int comparison and make sure 11 is bigger than 2 (float 5.10 == 5.1).

The SL upgrade process is typically these steps:

Install yum-conf-sl#x
Clean the package cache (pkcon refresh force)
The package manager now offers updates from the latest release

I'm not seeing anywhere how I can suggest that behavior to gnome-software.....

I'd love to use the auto expire bits of libxmlb to simplify the cache maintenance, but couldn't figure out how.

[1] Example: http://ftp.scientificlinux.org/linux/scientific/7/x86_64/os/repodata/repomd.xml