Permit easy creation of smart-variables

No longer restrict smart-var new and create.
Adapt smart-vars form to its possible contexts:
- from the /lookup_keys controller:
There is no context, hence puppetclass is freely modifiable.
- from the /puppetclass/edit|new controller:
Smart-vars are in a dedicated tab, hence there is no need to
display the current puppetclass, and would be forbidden to change it.
- from the /puppetclass/id/lookup_keys/edit controller:
We need to display the parent puppetclass,
but we forbid to change it, not to change the context.