Add populator_type column to spree_variants to set populator class.

Change-Id: Ie01bd20ed947c2477fda3599db4675e6033b5ccb
Reverse order of last arguments to get_available_keys.

Not all variants have license_key_types, so license_key_type should
be an optional argument and default to nil in case nothing is passed in.

Change-Id: I67f0104c80c0143e9bb19c6f66f204f92daf28d9