client/shared/distro.py: add Probe based on Python's Standard Library

Ross Brattain pointed us to the platform.dist() function on Python's
Standard Library. We realized it's valuable to use it in addition
to our custom Probes.

So this introduces a Probe that relies on platform.dist(), that will
have its result used if no other (more specific) Probe exists.

Signed-off-by: Cleber Rosa <crosa@redhat.com>