SPARK-3178 setting SPARK_WORKER_MEMORY to a value without a label (m or g) sets the worker memory limit to zero

Now the worker will fail fast if the memory is set to zero by leaving off the the label (m or g) either in the SPARK_WORKER_MEMORY environment variable or from the command line. 
Oh, to be more specific: you'll have to change the code that reads the environment variable to use `SparkConf.getEnv` instead of `System.getEnv`; I only changed this for the environment variables used in my specific test because I didn't want to make a big cross-cutting change across the codebase (plus it would probably get broken by subsequent PRs; we should add a style checker rule that complains about System.getEnv uses if we plan on doing this change globally).