Remove unused Foreman integration code

With a slight change in the way we're integrating foreman, some parts of our
code base were not useful anymore. This PR removes this parts.

Also some code that was used only by the Foreman integration code is removed as
well(e.g. AbstractModel). It doesn't mean we can't use it in the future but
that's why we have VCS. For now, it's better to not keep unused code in the
code base.

The only Foreman-related thing right now is a foreman_id attribute in users
table, since it's likely to be used soon. The whole katello-glue-foreman is
obsoleted now. The new integration code should be designed around Rails
engines, since it brings better separation of code-base comparing to packaging
based solution.

In katello-configure Foreman installation is optional, since right now, the
integration is not in a phase that it would be useful to have it by default.
However, for development purposes it's still possible to get Foreman and
Katello running on the same machine. The only thing is to install
katello-configure-foreman package. With it, the installation is the same as before.

It's also possible to install katello only first, and then add the
katello-configure-foreman package and install the Foreman later.