<yum-validator> Rename assets from *check-sources to *yum-validator

This set of changes prepares the code base for the next step, which is
reorganization for packaging. Changes include:

* `oo-admin-check-sources.py` is renamed to `oo_admin_yum_validator.py`
    * This will eventually be called from a wrapper script -
      `/usr/sbin/oo-admin-yum-validator` - to maintain consistency of
      naming with other `oo-admin-*` scripts
* Towards implementing a wrapper script, the main program logic for
  `oo_admin_yum_validator.py` is now in the module function
  `validate_yum`
* Class `OpenShiftAdminCheckSources` is converted to a new-style class
  named `OpenShiftYumValidator`
* Class `OpenShiftCheckSources` is converted to a new-style class
  named `CheckSources`