Set log_facility for keystonauth middleware

Otherwise, unconfigured keystoneclient logging triggers annoying
syslog messages:

$ swift --insecure list

Message from syslogd@d52-54-00-26-22-3a at Sep 13 13:47:02 ...
 <131>proxy-server STDOUT: No handlers could be found for logger
 "keystoneclient.common.cms" (txn: txe68451518fc146169ef57b4ee4d83fa5)