[feat] add fallback on trust sources for ssl verification

With the merge of platformTrust in twisted, the situation for cert chain
verification in linux improved a lot.

This patch implements fallbacks to do the following:

- Try to use whatever trust sources are found in the system. This means
that if ca-certificates is installed, pyopenssl will have a valid set of
root certificates and verification will likely work (twisted uses
platformTrust for this).

- If that fails, try to use certifi. We could/should depend on that from
now on, *but* it's not packaged before stretch.

- So, I'm not deprecating its usage right now, but this one should be
the last cacert.pem bundle that we ship with leap.common.

- If the cacert.pem from leap.common fails to be found, well, there's
nothing you can do. Your TOFU attempt with a cert coming from the
CArtel will fail.

Most of this MR should be sent as a patch upstream, see https://twistedmatrix.com/trac/ticket/6934
Also related: https://twistedmatrix.com/trac/ticket/9209

I think proper testing will depend on merging https://github.com/pyca/pyopenssl/pull/473

- Resolves: #8958
- Release: 0.6.0

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