Clean up in test_server_diagnostics unit test case

This patch changes V3 router to V21 router,
and HTTPRequestV3 to HTTPRequest. which can be shared between
V21 and V2

Partially implements blueprint v2-on-v3-api
Change-Id: Id3c91c377b9ba98c5cf2d1d2b4f476cec3159792
Merge "Clean up in test_server_diagnostics unit test case"