1017354: remove msg printed to stderr via yum

There was a message "This system is receiving updates from Red Hat
Subscription Management." printed by yum to stderr. This
is not an error msg, or a warning, but indicative of
correct behavior, so remove it.
Merge branch 'master' into alikins/1017354_yum_plugin_msg

Conflicts:
	src/plugins/subscription-manager.py
Merge pull request #879 from candlepin/alikins/1017354_yum_plugin_msg

1017354: remove msg printed to stderr via yum