Adds DELETE /v2/plans/:plan_uuid/:role_name/:role_version

This patch adds hollow
DELETE /v2/plans/:plan_uuid/:role_name/:role_version
endpoint which returns only dummy response.
Logic to manipulate real data will be added later.

Change-Id: If97195b3a1ac4f7a1003b04414742a03c946efd3
Implements: blueprint tripleo-juno-tuskar-plan-rest-api