Add support for Origin yum repos, de-Enterprise output

This commit adds support for yum repositories, Origin yum repo
definitions in `repos.ini`, and removes most of the references to
Enterprise from the program output. `README.md` has also been updated to
reflect the present state of the tool.

Notable changes include:

* `yumvalidator/check_sources.py`: `installed_package_matching` method
  added; simplifies checking for (obviously) installed
  packages. Resolves a logic error in `oo-admin-yum-validator` in the
  `guess_role` method.

* `oo-admin-yum-validator`: `guess_role` method is modified to use
  `CheckSources.installed_package_matching` to resolve bug where a
  role was assigned if its key package was available for install; role
  should only be assigned if the key package is actually installed.

* `oo-admin-yum-validator`: `guess_role` method updated to attempt to
  guess `RepoDB.RepoTuple.product` by inspecting repositories
  providing key packages.