code: def get_OS_UUID(cls, os):
          
          if os in cls.templates:
              return cls.templates[os]
  
          uuid_regexp = '^[0-9a-z]{8}-[0-9a-z]{4}-[0-9a-z]{4}-[0-9a-z]{4}-[0-9a-z]{12}$'
          if re.search(uuid_regexp, os):
              return os
  
          raise Exception((
              "Invalid OS -- valid options are: 'CentOS 6.5', 'CentOS 7.0', "
              "'Debian 7.8', 'Debian 8.0' ,'Ubuntu 12.04', 'Ubuntu 14.04', 'Ubuntu 16.04', "
              "'Windows 2008', 'Windows 2012'"
          ))
docs: Validate Storage OS and its UUID.
  
          If the OS is a custom OS UUID, don't validate against templates.