Disable Mistral

https://review.openstack.org/#/c/454710 broke us and now we need
https://review.openstack.org/#/c/455425/ to fix it, chicken and egg.

So let's disable Mistral until we get a promotion in RDO and re-enable
it again so we can land https://review.openstack.org/#/c/455425/ and
revert this patch and test Mistral again.

Change-Id: Ia7bf048901abf1cb96fa27e56752f66eac92f885

fixtures/scenario003.pp

@@ -26,7 +26,7 @@ case $::osfamily {
   }
   'RedHat': {
     $ipv6            = true
-    $mistral_enabled = true
+    $mistral_enabled = false
     $murano_enabled  = true
     $trove_enabled   = true
   }