This should deal with the CI port already in use warning situation reported in #4806

Highlights

• We have a few places where we start up a cluster (e.g. spec cluster or gen_cluster) where we do not close the scheduler if workers fail to start. This would leave ports still allocated
• Previously HTTP server sockets were opened during Scheduler.__init__ which made some tests really awkward. This PR moves the start of the HTTP server to the Scheduler.start method.
• A few tests intentionally start two clusters/schedulers where I circumvented this warning by simply allowing random ports. This revealed a shortcoming of our API since there is currently no way to not start an HTTP server
• I added a filterwarnings statement in our setup.cfg to err whenever this warning is raised in our test setup. This is very strict but I would prefer keeping it and removing it again if it causes any issues. Preferably we'd have a test fixture which checks if the test left any open socks but I'm not aware of any code which can do this without root

Supersedes #4896 and #4921

• Closes [CI] Warnings about used ports #4806
• Tests added / passed
• Passes black distributed / flake8 distributed / isort distributed