Use short form for references

Since documentation is now derived from the validation framework, it makes sense that our internal representation should match the user's ideal representation. When a command needs an attribute to reference an existing object, the code previously used a long form (`{"name": "value"}`) rather than the shortcut form (`"value"`). This corrects the internal representation without affecting the external.

For backwards compatibility, `create-policy` will retain the old format so it can create new tags.

Fixes https://tickets.puppetlabs.com/browse/RAZOR-246