Returns AMI name using Cirrus ami naming convention.

def AmiName(ami_release_name, ubuntu_release_name, virtualization_type,
            mapr_version, role):
  """ Returns AMI name using Cirrus ami naming convention. """
  if not role in valid_instance_roles:
    raise RuntimeError('Specified role (%s) not a valid role: %s' %
                         (role, valid_instance_roles))
  if virtualization_type not in valid_virtualization_types:
    raise RuntimeError('Specified virtualization type (%s) not valid: %s' %
                         (virtualization_type, valid_virtualization_types))
  ami_name = 'cirrus-%s-ubuntu-%s-%s-mapr%s-%s' % (ami_release_name,
                                                   ubuntu_release_name,
                                                   virtualization_type,
                                                   mapr_version, role)
  return ami_name