In order to deal with horizon going into the upper-constraints file so
that horizon plugins can consume it normally, we need to align the
constraints invocation in horizon's tox.ini. This means putting the -C
argument into the dependencies list rather than the pip install line so
that it doesn't get appended to tox installing horizon itself.
While we're in here, move nosehtmloutput into test-requirements so that
we don't need to do .[test] which also doesn't work with constraints.