Adds PATCH /v2/plans/:plan_uuid endpoint

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

Change-Id: Ia6b14ccc0834f833560dab2d2b188ea90dea1807
Implements: blueprint tripleo-juno-tuskar-plan-rest-api