Verify delete agent response of V2/V3 APIs

This patch adds the JSON schema for response status of Nova agent delete APIs
to block the backward incompatibility change in the future.

delete agent does not return the response body.
Response status of above APIs is below:
V2 : 200
V3 : 204

Partially implements blueprint nova-api-attribute-test

Change-Id: I1040b61b081d783a7c206189e11d16f9f9e77f91
Merge "Verify delete agent response of V2/V3 APIs"