Add blob_chunk_size to get_default_settings of GoogleCloudStorage (js…

…chneier#878)

Create `blob_chunk_size` as a `GoogleCloudStorage` attribute so that it's easy to override `GS_BLOB_CHUNK_SIZE` in custom Google Cloud storage backends. Without this change, you have to create a new `GoogleCloudFile` and copy a lot of code to make it work

storages/backends/gcloud.py

@@ -35,7 +35,7 @@ def __init__(self, name, mode, storage):
         if not self.blob and 'w' in mode:
             self.blob = Blob(
                 self.name, storage.bucket,
-                chunk_size=setting('GS_BLOB_CHUNK_SIZE'))
+                chunk_size=storage.blob_chunk_size)
         if self.blob:
             self._apply_backoff_blob()
         self._file = None
@@ -148,6 +148,7 @@ def get_default_settings(self):
             # rolled over into a temporary file on disk. Default is 0: Do not
             # roll over.
             "max_memory_size": setting('GS_MAX_MEMORY_SIZE', 0),
+            "blob_chunk_size": setting('GS_BLOB_CHUNK_SIZE'),
             "retry": setting('GS_RETRY', False),
             "initial_delay": setting('GS_INITIAL_DELAY', 1.0),
             "max_delay": setting('GS_MAX_DELAY', 60.0),