zdtm: use flake8 to verify zdtm.py

and fix various warnings. For example, we mix tab and space indentations.

v2: add flake8.cfg

Cc: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>