Make service_instance flake8 compatible

Change-Id: I62dabcc5c76359878f609a6040adcbfd17e8c5e6
Fix few issues in service_instance

Change creation order - previously there could be
situation where object would be created but it's resource_id was
unset resulting in it being impossible to delete by heat.

Change-Id: Iea196deaca34a7e4e465bfa1df21b1014246cc18