Set swift3 before s3token in proxy server middleware pipeline

swift3 and s3token middlewares were not set in correct order in
/etc/swift/proxy-server.conf when the swift3 service is enabled
with keystone. swift3 must be set before s3token.

Fixes: bug #1166123
Change-Id: I97c051af95cbdc26ccfe9753c5719394e1875dde