add capsule support to API/CLI

See http://projects.theforeman.org/issues/5821#note-1 for more details on how to use that.

Things to do before merging:

- [x] - authorization
- [x] - tests
- [x] - address PR issues
- [ ] - merge the PRs to katello-installer to fix the capsule installer http://projects.theforeman.org/issues/5815
- [x] - resolve missing v1 controller causing katello-agent http://projects.theforeman.org/issues/5804 to not be initialized