Allow reading and writing shared config file

We want to create a new config file that is shared across the different libraries and tools that use the gridscale API: https://github.com/gridscale/terraform-provider-gridscale/issues/183

For this we need to have a function that reads that config file into an internal representation and probably also a function that writes such a config file if it does not yet exists.

See https://github.com/gridscale/terraform-provider-gridscale/issues/183 for the format of the file and file paths on the different platforms.