Update magento-single.yaml

Tested on CentOS 6.5 (PVHVM). All appears well
update tests to handle CentOS
Make default image Ubuntu 12.04
use CentOS 6.5 plain until we add PVHVM support
update test error messages to be dynamic
also correct the debian family memcache user name
add test suite for CentOS 6.5
Merge pull request #6 from rackspace-orchestration-templates/add-centos-option

Add CentOS 6.5 option