This change updates the gnocchi::keystone::auth class to include a default
service_name of 'gnocchi' so that if a user changes the auth_name, the
service is still created as being related to 'gnocchi'. This improves the
user experiance when they want to customize the usernames for services.
Additional changes:
* update related spec test
* add a release note
* add the service_description option in order to configure it
Change-Id: Iaca2fe1ec33041593ddc8ee9b7bd7a0cd31f1f3f
Closes-Bug: #1590040