Modernize CloudStack instance credentials and security group code

See JCLOUDS-105 for a symptom caused by this - the CloudStack login code is rather archaic. It still expects the largely deprecated credentials store to be populated and the like. It should really get modernized to be like, say, openstack-nova. Same thing with security groups - we assume they already exist or we muck around with the default security group, and that's not optimal by a long shot.