tests/vm: change scripts to use self._config

tests/vm: change scripts to use self._config

This change converts existing scripts to using for example self.ROOT_PASS,
to self._config['root_pass'].
We made similar changes for GUEST_USER, and GUEST_PASS.
This allows us also to remove the change in basevm.py,
which adds __getattr__ for backwards compatibility.

Signed-off-by: Robert Foley <robert.foley@linaro.org>
Reviewed-by: Peter Puhov <peter.puhov@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200601211421.1277-8-robert.foley@linaro.org>
Message-Id: <20200701135652.1366-12-alex.bennee@linaro.org>