Set server.plugins value to valid value

[MJJoyce]

The default value provided for server.plugins in config.yaml is resulting in an unexpected None in some checks. While this one again brings up the idea of checking for defaults / expected values on an initial config load and warning appropriately, it's easier in the near term to make sure that if we set the value that it's valid.

The specific example where this is causing problems:

Our current server.config value

    server:
        plugins:

The code in GUI that is pulling this value breaks when server.plugins exists but isn't a valid form

        plugins = ait.config.get('server.plugins', [])
        for i in range(len(plugins)):

Removing server.plugins or setting it to an empty list fixes this problem.