storage manager: fix b2 storage bugs.

* B2 doesn't allow the path to start with a '/' character.
* When storing the file, we have to rewind the file pointer to make sure
  we get the whole file.

app/logical/storage_manager/cloud.rb

@@ -9,19 +9,24 @@ class StorageManager::Cloud < StorageManager
   end
 
   def store(io, path)
+    io.rewind # XXX caller should be responsible for this.
     data = io.read
-    client.put_object(bucket, path, data)
+    client.put_object(bucket, key(path), data)
   end
 
   def delete(path)
-    client.delete_object(bucket, path)
+    client.delete_object(bucket, key(path))
   end
 
   def open(path)
     file = Tempfile.new(binmode: true)
-    response = client.get_object(bucket, path)
+    response = client.get_object(bucket, key(path))
     file.write(response.body)
     file.rewind
     file
   end
+
+  def key(path)
+    path.delete_prefix("/")
+  end
 end