Several softwares were using fixed versions, but versions are also defined in
stack/slapos.cfg, so let's use from stack/slapos.cfg which will simplify
version management.

This urllib3 has visible changes for us, it no longer send the request path
"as is", but normalizes the . and .. which is not what we want in
caddy-frontend test, so we adjust the test to use prepared requests, which is
the recommended approach for this kind of low level needs. Since prepared
requests does not set default user agent "python-requests", tests checking that
user agent is in log files have been adjusted to use a dedicated test user agent
header.

Another change is that certificates are now verified by default, so we adjust
seleniumserver tests to disable certificate validation. jstestnode, already
had a patch regarding certificate validation, because we wanted certificates
to be validated before it was the default. We just update this patch a bit
because we want to keep the optional behaviours of validating against a
specific CA or to disable validation.