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>
This new helper doesn't seem to provide much benefits over using [BuiltIn.Length Should Be](http://robotframework.org/robotframework/latest/libraries/BuiltIn.html#Length%20Should%20Be).