Support passing max-bandwidth and max-concurrency options for node synchronization.

Summary of changes:

Distributor:
- Convert to use importer *standard* configuration keywords when specifying the ssl client certificate in the bind payload.

Importer:
- Convert to use standard importer configuration keywords for (ssl cert, max_concurrency and max_bandwidth)
- Use importer_config_to_nectar_config() instead of creating nectar download configuration by hand.

Agent handlers & model:
 - Add support for passing max_concurrency and max_bandwidth options as override_config in the repository sync REST request.

Admin Extensions:
 - Add support for max_concurrency and max_bandwidth options in both regular commands and scheduled commands.

Update unit tests for all of this :)