Support predefined repos in the default configuration

In the default configuration, the repositories for an organization is an empty list. However, it is useful to define some defaults there that should be created by otterdog, i.e. the .eclipsefdn-private repo with certain default settings. This would allow us to simplify the setting up of a new organization.

Due to the modelling of the default configuration (repositories are an array), it will not be possible to override settings for repos already contained in the default config as jsonnet does not provide an inheritance mechanism for array elements.