Adds POST /v2/plans/:plan_uuid/roles endpoint

This patch adds hollow POST /v2/plans/:plan_uuid/roles
endpoint which returns only dummy response.

Logic to manipulate real data will be added later.

Change-Id: I6ffac549b2a0ff240a8500f7c0644d50b270f77a
Implements: blueprint tripleo-juno-tuskar-plan-rest-api