Add whitelist entry for s-proxy 'Timeout talking to memcached'

According to John Dickinson:

"This isn't an error. It's logged at an error level because it may hint
at other problems, and it's something an operator needs to know about,
but it is not an unhandled failure condition. (Think of it similarly how
in swift a server being down is an "error", but it's something that
swift seamlessly works around.)"

Closes-Bug: #1260894

Change-Id: I41e55c5e34ee214727fbbd7b9daa1f6ea9bf8050
1 file changed