Added tasks to handle basic registration task.

This pull request added a few tasks related to the process of
registering, subscribing and then consuming content from an existing
Satellite 6 instance.

The following environmental variables are expected when calling the main
task, `client_registration_test`:

* RHN_USERNAME: Valid username for Red Hat RHN/CDN.
* RHN_PASSWORD: Valid password for Red Hat RHN/CDN.
* ORG: Name for an existing organization (defaults to 'Default_Organization').
* ACTIVATIONKEY: Name for an activation key
* CERT: The URL for a Satellite 6 cert rpm.

I have also modified the `subscribe` task by making the subscription
against a `pool id` optional and adding the new `autosubscribe`
parameter (defaults to False).
What about s/CERT/CERTURL?