* Python: avoid assigning same port number to multiple grpc sessions
* Add retry count when checking for port number file
* Use grpc to find and use port number
* Add test used to start several instances of resinsight at the same time
Testing up to 50 instances works well
* Python: allow launch_port == 0 to assign port by GRPC in Instance.launch().
Also allow longer wait before failing the port number file reading:
it can take some time to launch when launching lots of instances at
the same time.
