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.
Merge pull request #110 from dobbymoodge/check-sources

Update workflow for more sensible yum-plugin-priorities advice
<install> bind_key; preflight checks; docs

* added some preflight sanity checks like "is this RHEL 6?"
* compressed some verbose code defining yum repos
* after some research, discovered that the bind key is not half of a
keypair and can be set arbitrarily; re-enabled that ability.
<install> use 2.0 beta channels from RHN
Merge pull request #111 from sosiouxme/enterprise-2.0

improvements + beta channels
<oo-admin-check-sources.py> Bug 1023359, detect disabled RHEL channel and language tweaks

Updated required_repos routine to include RHEL base channel in
results, fixing 1023359.

Replaced both vague and overly-specific RHSM and RHN subscription
advice with link to KB article. The article has instructions for
attaching RHSM or RHN subscriptions for broker and node
systems. Instructions for OSE 2.0 will be included when they are
available.

Made available package detection more efficient by switching to
YumBase.searchGenerator()
Merge pull request #112 from dobbymoodge/check-sources

<oo-admin-check-sources.py> Bug 1023359, detect disabled RHEL channel and language tweaks