This is a refactor of the Provider schema templates and generation
process to make more reuse of common types and structures:

* The repeated structure from outside wrapper has been refactored into
  templates/provider/endpoint.json

* Common vehicle information (provider_id, vehicle_type, etc.) has been
  refactored into a defintion in templates/common.json, which is added
  to the list of definitions for each endpoint

* Each endpoint specific template has been refactored to contain only
  those properties and definitions specific to the endpoint; common
  properties and definitions are merged at generation time

* The generation script was improved for readability and runtime feedback

* The generation script accepts arguments for selectively generating
  Agency and/or Provider schemas (default is still to generate both)

* reuse common string definition in Agency templates

* renaming vehicle fields in Agency to make common schema reuse easier

* renaming status_changes.associated_trip --> status_changes.trip_id
  and reusing validation rule in Agency and Provider schemas

* pluralizing propulsion_types (array field) in both

* further reorganize schema generation for alignment between Agency and
  Provider

* use new field names and plurality
* common state and event defintions merged by schema generator
* valid state/event combinations merged by schema generator

Existing endpoints are being augmented with Docked information
and new endpoints will co-exist.

Ensure that the vehicle_events array contains
at least one of the valid transition events
for the given vehicle_state