Add "default" speeds to mjolnir config

GraphEnhancer sets default speeds (in the UpdateSpeeds method) to edges that do not have max_speed tags. These values are hard coded right now. We should add default values to the config file. This request was also made to allow someone testing a small region (e.g. a US state) to use custom values that might be better for that admin region.

Ultimately we should have admin (at least country) specific defaults for each highway tag/road classification.