python-holdup (or similar) for wait-on-login

[joshmoore]

If we begin to have other python requirements for running test-infra (perhaps even via pip install omero-test-infra), we might look at https://github.com/ionelmc/python-holdup to replace some of the code from https://github.com/openmicroscopy/omero-test-infra/blob/master/wait-on-login#L29