Make v2.1 tests use wsgi_app_v21 and remove wsgi_app_v3

Nova v3 API has disappeared but some tests still access v3 URLs.
This patch makes them use wsgi_app_v21 and access to v2.1 URLs.
In addition, this patch removes wsgi_app_v3 because it is not
used after this change.

Partially implements blueprint v2-on-v3-api

Change-Id: Ic36d86dce8a35933c15ec820b273825bb0353864
Merge "Make v2.1 tests use wsgi_app_v21 and remove wsgi_app_v3"