Add signing dir

Currently, my swift installation fails with the following
error message:

  [Errno 13] Permission denied: '/root/keystone-signing

and the proxy server can never service requests.

This is b/c by default the client side directory for
pki-signing seems to use $HOME. This commits sets the
directory to be /var/cache/swift/ and resolves my
swift-proxy issues.

Change-Id: Ice904a9d53432ba1a1ae9516cd0ee6204ade63bb