Allow users to choose the version of Swift to use

Fixes #442

storage/swift.py

@@ -22,7 +22,11 @@ class SwiftStorage(BaseStorage):
     self._swift_user = swift_user
     self._swift_password = swift_password
 
-    self._auth_version = auth_version or 2
+    try:
+      self._auth_version = int(auth_version or '2')
+    except ValueError:
+      self._auth_version = 2
+
     self._os_options = os_options or {}
 
     self._initialized = False