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>
Maybe if we would create a helper for verifying the message count, we would not need the comments? (The name of the test case should give us an idea what is happening?)

Verify message count   [Arguments]    ${count}    ${message list}
     ${length}=       Get length     ${message list}
     Should be equal as integers      ${length}     ${count}