Extend Activation Keys to support System Purpose
As we have done for Content Hosts, we will use the functionality of Candlepin to support System Purpose on Activation Keys.
This feature should ensure that:
- Katello::ActivationKey model supports 'purpose_usage', 'purpose_role', and 'purpose_addons' just like Hosts
- Existing functionality does not regress
- Activation Keys API supports setting/modifying/etc the purpose attributes
- Activation Keys can be searched by their syspurpose attributes