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>
1) This example uses the same value that is the default for the argument. Wouldn't it be better to use something else, for example, `NONE` to disable logging altogether?

2) This new option should also be at least briefly documented in this docstring too. The doc should contain a link to `Logging` section and note about the option being new in Robot Framework 2.8.7.