Set permissions on generated ring files

The use of NamedTemporaryFile creates rings with permissions 0600;
however most installs probably generate the rings as root but the
swift-proxy runs as user swift.

Set the permissions on the generated ring to 0644 prior to rename so
that the swift user can read the rings.

Change-Id: Ia511931f471c5c9840012c3a75b89c1f35b1b245
Closes-Bug: #1302700