Fix status code of server's action confirm_resize for v3

The status code of confirm_resize should be 202, but it
returns 204 now. That because it returns HTTPNoContent
that overwrites the status code of decorator 'wsgi.response(202)'

And adds API samples for confirm_resize that used to test
the status code of confirm_resize.

Partially implements blueprint v3-api-unittests
Closes-bug: #1233561

Change-Id: I50a3046866af0a1efdb5c41ded40f973b02843e8
Merge "Fix status code of server's action confirm_resize for v3"