Tweak model for Hnic

- The headnode field is now non-nullable
- The constructor now takes a 'group' argument
Add API calls headnode_{create,delete}_hnic
Add unit tests for headnode_{create,delete}_hnic
Add REST wrappers for headnode_{create,delete}_hnic
Add CLI wrappers for headnode_{create,delete}_hnic
Merge pull request #136 from gsilvis/api-headnode-create-delete-hnic

Api headnode create delete hnic