This allows users to set the defaultConfigurationName project wide. This
setting corresponds to the drop down in the project settings that says
"Use CONFIG for command line builds". This affects which configuration
Xcode looks in for some settings, even if you pass `-configuration FOO`.
This gives us an entry point for disabling specific project.yml
validations that sometimes shouldn't apply. The first example is
missingConfigs, which, if you use your .yml file in multiple projects,
can be too strict if the projects have different top level
configurations.