functional tests do not exit properly due to keystone_paused running config-changed after all apps removed from model

[Drew Freiberger]

Functional tests for 21.01 cycle are passing, but are hanging on model tear-down due to race condition between pytest's model tear-down and the fixture keystone_paused config-changed after the 'yield keystone'.

Traceback shown here:
https://pastebin.canonical.com/p/dqVvS2VScz/
Model status here:
https://pastebin.canonical.com/p/jvVcnf6rDn/

The config-changed to revert the service-port in the keystone_paused fixture needs to be skipped if the model is being torn down, as the config-changed errors and the functest model hangs indefinitely.

Workaround is to 'juju remove-machine --force 0' from the functest-XXXXX model after all tests have passed.