disable pycontracts except in development environments (PLAT-122)

@cpennington 
@dmitchell 

https://openedx.atlassian.net/browse/PLAT-122

This should disable pycontracts in workers and management commands when the feature flag is enabled. I wasn't sure if it should be enabled in testing environments.
You mean submit something upstream? Check for the env variable here? https://github.com/AndreaCensi/contracts/blob/master/src/contracts/enabling.py#L20