Allow updating extra Application Plan values from system API

Using the Application plan update endpoint I can't modify those fields:

• trial_period_days
• cost_per_month
• setup_fee
• end_user_required
• cancellation_fee

Code of the application plans controller:
https://github.com/3scale/porta/blob/85c608d6bc0764166e755eeb49310886b092596a/app/controllers/admin/api/application_plans_controller.rb#L133-L141

jprusi@redhat.com cancellation_fee is not an existing attribute