Add cloud-user to non-root user list
Add cloud-user to non-root user list

RHEL and CentOS cloud images use 'cloud-user' as the default user for
login. This adds support for trying that user if root does not work.

Change-Id: Ie9e4276800408dee5941971d49da35252f9a74ee

Tesora/tesora-nodepool,Tesora/tesora-nodepool,seandst/nodepool,citrix-openstack/nodepool,seandst/nodepool,citrix-openstack/nodepool