Pin puppet-rabbitmq to 2cd0fb2abc001a76e7e35446231773e0e54c1ff8

With voxpupuli/puppet-rabbitmq#797
merged e90dcbdeb3e8 which removed params.pp and server.pp
Let's pin to the previous commit until this gets sorted
out more carefully

Example of a broken job:
https://logs.rdoproject.org/openstack-periodic-master/opendev.org/openstack/tripleo-ci/master/periodic-tripleo-ci-centos-7-ovb-3ctlr_1comp_1supp-featureset039-master/0109c60/logs/overcloud-controller-0/var/log/journal.txt.gz#_May_13_03_24_41

Change-Id: I72306636fd7e49169790801f4fe5683bd0fabf0b

tags/train-uc.yml

@@ -831,6 +831,7 @@ packages:
 - project: puppet-rabbitmq
   tags:
     train-uc:
+      source-branch: 2cd0fb2abc001a76e7e35446231773e0e54c1ff8
 - project: puppet-redis
   tags:
     train-uc: