Refactoring KG_WHITELIST to KG_ENV_WHITELIST

To better align with Jupyter Kernel Gateway, this commit will change from
KG_WHITELIST to KG_ENV_WHITELIST and use comma as a delimeter instead of a colon.

nb2kg/nb2kg/managers.py

@@ -113,7 +113,7 @@ def start_kernel(self, kernel_id=None, path=None, **kwargs):
                 method='POST',
                 body=json_encode({
                     'name': kernel_name,
-                    'env': {k:v for (k,v) in dict(os.environ).items() if k.startswith('KERNEL_') or k in os.environ['KG_WHITELIST'].split(":")}
+                    'env': {k:v for (k,v) in dict(os.environ).items() if k.startswith('KERNEL_') or k in os.environ['KG_ENV_WHITELIST'].split(",")}
                 })
             )
             kernel = json_decode(response.body)