Ensure that only valid environment names are created

Since the only constraint on Application Names is <100 characters, but the constraints on environments are tighter (letters, numbers, dashes, 23 characters, see http://docs.aws.amazon.com/elasticbeanstalk/latest/APIReference/API_CreateEnvironment.html), the createEnvironment task could let junk through without a way to correct it.

This PR throws a tiny notice and then adjusts the return if we would otherwise attempt to create an invalid environment name.