cmd-kola: transparently pass unknown flags to kola

[jlebon]

Right now we're kind of in a messy situtation where we have some args
for cosa kola, and other args that we want to pass through to kola.
Unfortunately, argparse isn't very good for this use case. Using --
helps sometimes, but the issue arises when we want to pass flags to
both cosa and kola. The obvious --cosa-arg -- --kola-arg doesn't
work (https://bugs.python.org/issue15112).

Instead, just use parse_known_args() to tell argparse to leave the
unknown args untouched. We then pass those through as is to kola. This
also avoids us having to use -- entirely. The caveat is that it can be
confusing if we support a cosa kola arg that kola natively supports,
but let's say we're not going to do this, or if we do, it's to just pass
it through to kola as well as do some auxiliary related thing.

[jlebon]

Another gotcha with this approach is that you want the kola command to come first. E.g. cosa kola run [--cosa-or-kola-arg...], not e.g. cosa kola --my-arg-1 run --my-arg-2. Otherwise, there's no way for argparse to know if the positional arg is actually an argument to an unknown flag.

[jlebon]

(One related thing I've been thinking about recently though is that we should just thin out the cosa kola layer and teach kola directly about cosa concepts. That'll allow us to fold things like the install test into kola more easily.)

[cgwalters]

Eventually I think we want kola to really do the right thing out of the box.
But, this is #163