dockertest.dockercmd: Support debug cmdline option to debug docker cmds

This patch makes it possible to globaly turn off/on logging of docker
commands. Logging docker cmds is essential for me as QE in order to
be able to reproduce the situation and make additional steps to understand
the problem.

It's still possible to disable verbose but it should be used only when
you log the command yourself (eg. in case of long iterations)

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>