Support programmatic configuration variables

[mahic]

The library supports setting configuration variables such as DATACONTRACT_DATABRICKS_TOKEN via os.environ
In a multithreaded environment, this will cause race condition issues and security issues if the token supplied is a user token.

The workaround currently is to invoke subprocesses with env vars set for each sub process, but it would be a far better solution if one could set configuration vars programmatically, ie as a param on the DataContract type.

[jschoedl]

Sounds good!

We plan to change our checks library soon (from Soda to Ibis). This might be a good opportunity to add this, because we'll touch each testing backend anyways.