Convert v3 server diagnostics plugin to v2.1

This patch changes v3 server diagnostics plugin to v2.1 and makes v2
unit tests share between v2 and v2.1.

This patch also delete the V3 server_diagnostics API tests.

The differences between v2 and v3 are described on the wiki page
https://wiki.openstack.org/wiki/NovaAPIv2tov3.

Partially implements blueprint v2-on-v3-api

Change-Id: If10615f3f119dda815d56cf8329619bab502e091
Merge "Convert v3 server diagnostics plugin to v2.1"