Rename debug testenv to demo and use sample config

Switch the tox testenv name for invoking the daemon to demo, and
also switch it to implicitly using the default sample configuration
in etc/mudpy.yaml rather than explicitly using the daemon config
from the test fixtures. This makes it a little more friendly for
developers and/or first-time users who want to try things out since
`tox -e demo` will get them the log streaming to stdout and the
process bound to the invoking terminal where they can terminate it
with a simple sigint. For those who want to use a different
configuration, it can be passed with a command-line argument, like
`tox -e demo mudpy/tests/fixtures/test_daemon.yaml` (as an example
to replicate the old "debug" behavior).