creator_username not configurable

[robbytobby]

Currently dtool uses the $USER environment variable to set the value for "creator_username" in the .dtool/dtool file of the dataset.
It would be very desirable to be able to configure the variable "creator_username":
Often people use dtool from different machines where they do not always use the same login name. This leads to the result that they appear as multiple creators in the lookup server, which is kind of confusing.