Migration to single ONOS SSH connection

In robot library 'onos.robot' there are a lot of keywords which setup a new SSH connection to onos for each call. This leads more an more to SSH connection issues while pipeline tests as well as individual tests in local environments. There exitsts already the mechanism to establish once a SSH connection to onos and use this for all interworking with onos. The mechanism already does a reconnect when connection is broken or timed out.

All keywords using an own established SSH connection have to be replaced by using single SSH connection. Set up and close of single SSH connection should take over by Common Suite Set up and Suite Teardown.