Rename methods to more appropriate names

#subscribe to #enable_repo
#subscribed_products to #enabled_channels
Add Rhn#available_channels with spec
Add Rhn#disable_channel with spec
Add Rhn#all_repos and spec
Add SubscriptionManager#enable_repo and spec
Add SubscriptionManager#disable_repo and spec
Add SubscriptionManager#all_repos and spec
s all
Add Rhn#enabled_repos
Add SubscriptionManager#enabled_repos