[foretello] new candplepin feature to modify content sets on registered systems

When hosts register and subscribe to foretello, the file /etc/yum.repos.d/redhat.repo is updated with the available content sets

name=Katello Nightly

Candlepin now has the ability to control the details of these content sets from the server on a per host basis. Thus a host registered and subscribed with the above could have that content set specifically disabled (enabled=0). Since the state of all the content sets are known by foretello, other interesting things could also happen such as determining that the $releasever of the host is 6.5 but only 6Server content is available in the host's registered content view.

At the moment, the scope of my design was simply to display the content sets available in subscriptions (with their content sets) in both the Subscriptions and System pages and allow enable/disable of them individually. ie. Present the redhat.repo representation server-side and allow manipulation there.

@jsherrill suggested that more could/should be done around this feature. For example, a foretello custom product could be made that included both 32 and 64 bit content rather than having to make two separate products, as is the case today.

Suggestions welcome. I'll capture the initial conversation in a wiki to base further design off of.

··· -- @thomasmckay

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