Allow disabling default scratch org configs

[jlantz]

Currently, every project will always have the 4 default scratch org configurations. We should support projects that want to disable one of the 4 default scratch org configs with the following yaml syntax in the project's cumulusci.yml file:

orgs:
    scratch:
        dev:
            config_file: None

[koenwesselman]

Hi @jlantz , figured I'd have a look at a ticket or two after our call yesterday to familiarise myself with the inner workings of CCI. Would you be able to create a feature branch for this one for me to create a PR towards? I believe that's the process you'd like to stick to? :-)