Fix the gulpfile for test

  In order to make travis CI working without the api.json file
  (which is for dev only), the proxy setting has been move into the gulpfile
  and is only loaded for the dev task.

  Also add instructions in README.md for dev environment.

  Refactoring of api.json.example in order to simplify the name of URLs
  (keystone instead of identity, keystone_admin instead of identity_admin).