Allow the ca package to be excluded to support registering with RedHat

[treydock]

My systems don't (yet) use Satellite or Katello but instead register directly with RedHat. This change is one of possibly many to allow the subscription_manager class to be used for registration directly with RedHat.

[waveclaw]

If you are using RHN directly you should be using the raw types and not the convenience class.

This is a major breaking change to the API for users that do use the convenience classes so I am rejecting it for the 4.x release.

It may be considered for a 5.x release but this breaks the public API with no documentation to that effect.

[waveclaw]

Since we are going forward to a new API in 5.0 I will add this now.

The documentation still cautions you to use the raw types with RHN and not rely upon the convenience of this module.