Support cluster configurations in --export-config mode

As is mentioned in MXS-4275, the --export-config option will not work if a cached cluster configuration is present nor will it read it from the cluster when starting up. The fix to MXS-4275 will cause the static configuration to be exported even if there would be a newer one in the cluster.

Adding clustered configuration support for --export-config requires two things:

1. Produce a INI file from the cached JSON file stored on disk: This is relatively straightforward to do as the mxs::config::Configuration class already produces the required output and the objects are constructed at the point of the export.
2. Communicate with the cluster on startup: this is slightly trickier to do as the current update mechanism is asynchronous and won't trigger an update until after all the workers have been started. This might require addition of special code for this specific purpose.