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.

Fixes #1879.
Merge pull request #1875 from hthiery/master

Telnet: enable logging level config option in TelnetLibrary for python telnetlib

Thanks to @hthiery for contributing this feature!