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)
use node instead of proposal for attributes
hide username from ui
Merge pull request #149 from MirantisDellCrowbar/bug/smoke/roxy/master

Swift Dispersion Report fix
Merge pull request #148 from vuntz/from-pebbles

Set log_facility for keystonauth middleware (cherry-pick from pebbles)
Add config value for replication interval
Add migration for replication_interval
Merge pull request #154 from saschpe/release/roxy/master-replication

Add config value for replication interval
python-keystoneclient needed for keystone authentication
Merge pull request #156 from jsuchome/require-python-keystoneclient

+1; it got smoketesed => merging.
declare dispersion_coverage as float

and decrease default value to avoid creating heavy swift load