The earlier code behaved exactly as though this default had been set,
but it was actually handled as a(n unnecessary) special case inside the
connection plugin, rather than set as an explicit default.

If the default is overriden either in ansible.cfg or the environment,
the new code will continue to work (in fact, it won't know or care,
since it just uses the value set in the PlayContext).

This is submitted as a separate commit for easier review to address
backwards-compatibility concerns.