Update workflow for more sensible yum-plugin-priorities advice

oo-admin-check-sources.py now refrains from telling the user to
install yum-plugin-priorities until a repository is found that carries
the package. The package is provided by the OSE repos, so from a blank
system, the tool guides the user through the following steps:

* Initial run
 * `oo-admin-check-sources.py`
* Select a version of OpenShift
 * `oo-admin-check-sources.py -o 1.2`
* Select one or more system role(s)
 * `oo-admin-check-sources.py -o 1.2 -r node -r node-eap`
* Register a subscription that provides the needed repos
 * `subscription-manager attach --pool=xyz`
 * `oo-admin-check-sources.py -o 1.2 -r node -r node-eap`
* Enable the needed repos:
 * `subscription-manager repos --enable=repo_id`
 * `oo-admin-check-sources.py -o 1.2 -r node -r node-eap`
* Install yum-plugin-priorities
 * `yum install yum-plugin-priorities`
 * `oo-admin-check-sources.py -o 1.2 -r node -r node-eap`
* Set appropriate repository priorities
 * `oo-admin-check-sources.py -o 1.2 -r node -r node-eap --fix`

In order to perform the package search during the yum-plugin-priorities verification step, `_init_yumbase` in `check_sources.py` needed to be updated to set a command line in the `YumBase.plugins` object via  `setCmdLine`. This works around a missing attribute error in the yum security plugin.