<yum-validator> Rewrite ver/subscription guessing to be generic and configurable

* Added `_available_subs` helper method to `oo-admin-yum-validator`:
  checks available repositories to determine which subscription
  management tools might be available and generates a hash with the
  results

* Added `detect_prod_version_and_sub` to `oo-admin-yum-validator`,
  replacing `guess_ose_version_and_subscription` method. This uses the
  available `RepoDB` data - including the `preferred_version` and
  `preferred_subscription` config values - to resolve
  version/subscription detection when multiple possibilities exist.

* Added logic to `repo_db.py` to support the new `preferences` INI
  section and set `preferred_subscription` and `preferred_version`
  variables for use in version/subscription guessing.

* Added `preferences` section to `repos.ini` to set preferred OSE
  version to 2.0 and subscription type to `rhsm`

* Removed old `guess_ose_version_and_subscription` method from
  `OpenShiftYumValidator`

* Added `RepoDB.list_available` to get sorted set list of a `RepoDB`
  attributes (needed for updated `validate_version` method in
  `OpenShiftYumValidator`)

* Updated `--help` output to remove valid arguments for `--oo-version`
  option

* Updated `OpenShiftYumValidator.validate_version` to check argument
  to `--oo-version` option against available versions in `RepoDB`
  object `self.rdb`

* Updated `OpenShiftYumValidator.main` to add `validate_version` as a
  requirement for running checks

* Merged `beta2.ini` and `repos.ini`

* Updated `oo-admin-yum-validator.txt2man` manpage to explain updated
  `--oo-version` argument and to tweak some wording, style and typo
  issues