Telnet: enable config option in TelnetLibrary for python telnetlib

To be able to change the logging behavior of the underlying python
telnetlib a new parameter is added in the open connection keyword.

Signed-off-by: Heiko Thiery <heiko.thiery@kontron.com>
Why checking against `None` here? Is there a possibility that it could be passed here otherwise than user explicitly using `${None}` on test data? If not, I don't think we need to take that into account.