Use v2.1 URLs instead of v3 ones in API unit tests

Now v2.1 API unit tests use fake.wsgi_app_v3() and they specify v3 URLs.
This patch adds wsgi_app_v21() for using v2 URLs in v2.1 API tests and
changes URLs in unit tests.

Partially implements blueprint v2-on-v3-api

Change-Id: I5cdd7dd1d435ab0e4e093168f01e96f8dac52e8c
Merge "Use v2.1 URLs instead of v3 ones in API unit tests"