[s3] add support for setting "cloudfront_key_id" and "cloudfront_key"…

… through OPTIONS (#1274)

storages/backends/s3boto3.py

@@ -257,6 +257,9 @@ class S3Boto3Storage(CompressStorageMixin, BaseStorage):
     config = None
 
     def __init__(self, **settings):
+        cloudfront_key_id = settings.pop("cloudfront_key_id", None)
+        cloudfront_key = settings.pop("cloudfront_key", None)
+
         super().__init__(**settings)
 
         check_location(self)
@@ -281,6 +284,9 @@ def __init__(self, **settings):
         if self.transfer_config is None:
             self.transfer_config = TransferConfig(use_threads=self.use_threads)
 
+        if cloudfront_key_id and cloudfront_key:
+            self.cloudfront_signer = self.get_cloudfront_signer(cloudfront_key_id, cloudfront_key)
+
     def get_cloudfront_signer(self, key_id, key):
         return _cloud_front_signer_from_pem(key_id, key)