[orchestration] make auth_encryption_key an attribute

[Ethan Lynn]

For now, auth_encryption_key can not configure by cookbook.
It should be a random string of length [16, 24, 32]

https://github.com/openstack/heat/blob/1dbb189270cffc0edf0721815582b39ab2776031/heat/common/config.py#L318-L321

cfg.StrOpt('auth_encryption_key',
               secret=True,
               default='notgood but just long enough i t',
               help=_('Key used to encrypt authentication info in the '
                      'database. Length of this key must be 16, 24 or 32 '
                      'characters.'))

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/cookbook-openstack-orchestration ocata-eol release.