Using set_host_overrides() in the connection plugin to access the ssh
argument variables from the inventory didn't see group_vars/host_vars
settings, as noted earlier. Instead, we can set the correct values in
the PlayContext, which has access to all command-line options, task
settings, and variables.

The only downside of doing so is that the source of the settings is no
longer available in ssh.py, and therefore can't be logged. But the code
is simpler, and it actually works.

This change was suggested by @jimi-c in response to the FIXME in the
earlier commit.