subtests.docker_cli: Fix rmi and tag indentation

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
dockertest.subtest: Modify the log_xn to log a warning on hazard

Set default execution route by using try/catch and log warning when
no arguments are passed and format characters are used.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>