Make dnf default for fedora, and systemd default for ubuntu/debian

Currently, we maintain a list of platforms that dnf and systemd are default for. This is unsustainable, as we have to update this list for every new release. What we should be doing is have a blacklist defining which subset of platforms the older provider is default for (yum and upstart, respectively). This will reduce the long term maintenance costs for these two providers.

We already have something like this for upstart, where we have a list of ubuntu platforms that upstart should be the default service provider for. However, the tests fail when we remove the platform restrictions for ubuntu systemd. Older ubuntu versions like 12.04 show that systemd is the default, which is wrong. We need to be careful this does not happen.