'returns a list of comma separated zones parsed from the GCE_ZONE environment variable.
If provided, this will be used to filter the results of the grouped_instances call'

def parse_env_zones(self):

    import csv
    reader = csv.reader([os.environ.get('GCE_ZONE', '')], skipinitialspace=True)
    zones = [r for r in reader]
    return [z for z in zones[0]]
