Fix flake8 complaints - F401
F401 'dnf' imported but unused
F401 'imp' imported but unused
F401 'os' imported but unused
F401 'subprocess' imported but unused
F401 'sys' imported but unused
F401 'yum' imported but unused
JIRA: COMPOSE-4108
Signed-off-by: Haibo Lin <hlin@redhat.com>