[s3] Re-allow overrides of access_key_names (#1026)

* Re-allow override of access_key_names, etc.

This change:

b13efd9

swapped from using `self.access_key_names` to `S3Boto3Storage.access_key_names`; it's unclear if that was intentional or not, but that means it does not accept overrides; we relied on overrides to use some bucket specific keys provided by rook ceph.

* Update s3boto3.py

storages/backends/s3boto3.py

@@ -395,16 +395,16 @@ def _get_access_keys(self):
         provided in the settings then get them from the environment
         variables.
         """
-        access_key = self.access_key or lookup_env(S3Boto3Storage.access_key_names)
-        secret_key = self.secret_key or lookup_env(S3Boto3Storage.secret_key_names)
+        access_key = self.access_key or lookup_env(self.access_key_names)
+        secret_key = self.secret_key or lookup_env(self.secret_key_names)
         return access_key, secret_key
 
     def _get_security_token(self):
         """
         Gets the security token to use when accessing S3. Get it from
         the environment variables.
         """
-        security_token = self.security_token or lookup_env(S3Boto3Storage.security_token_names)
+        security_token = self.security_token or lookup_env(self.security_token_names)
         return security_token
 
     def _clean_name(self, name):