Adds DELETE /v2/plans/<plan-uuid> endpoint

This patch adds hollow DELETE /v2/plans/<plan-uuid> endpoint
which returns only dummy response.
Logic to manipulate real data will be added later.

Change-Id: I5ce3764e0a50e5e83438b576fe7768e70d65d0a8
Implements: blueprint tripleo-juno-tuskar-plan-rest-api