Fixes and _parameter helpers in LookupKey

Simplify .select([fields]) with .all.
Fix a small error with new parameters in new classes
Fix a small error using value instead of default_value

Use _parameter helpers in LookupKey,
permits to separate the puppetclass from the parameter
and be agnostic to the actual key formatting ("class/param")

Allow lookup_keys.puppetclass_id to be NULL,
to allow disassociation of obsolete smart-varaibles.