change prefs.cloudUserName to PrefCloudStage.cloud_storage_email.
This is slightly more effective (direct instead of indirect), and
reduces the use of prefs.
This is part of limiting the use of prefs. (which is a precessor to
qPref) to the login process, and ultimately remove it.
Signed-off-by: Jan Iversen <jani@apache.org>
161bedf58d