conpaas image creation scripts: fix critical issues in register-image-ec2-ebs.sh

- include 40_custom, without which the resulting AMI won't boot
- really pass --snapshot to euca-register, without which euca-register fails
- add timestamp to ami_name to avoid errors because of default name conflicts
- include error message in case euca-register fails



git-svn-id: svn://svn.forge.objectweb.org/svnroot/contrail/trunk/conpaas@5548 ef35cc4a-2640-4bbf-98bc-65920db7d8ae