Pin puppet version 5.5. for CentOS beaker jobs

We merged [1] which now makes sure we use actually
latest puppet version and not latest 5.5 version.

I however forgot that we need to pin puppet version
5.5 for CentOS jobs since it doesn't support the
required Ruby version for Puppet 6.

[1] https://review.openstack.org/#/c/618920/

Change-Id: Id5ebc4ff8841768cdad756b847403e54ea24e188

zuul.d/base.yaml

@@ -175,6 +175,7 @@
     nodeset: centos-7
     vars:
       nodepool_type: centos7
+      puppet_gem_version: 5.5
 
 - job:
     name: puppet-openstack-beaker-ubuntu-bionic

zuul.d/beaker.yaml

@@ -29,6 +29,7 @@
       - ^.gitignore$
     vars:
       nodepool_type: centos7
+      puppet_gem_version: 5.5
 
 - job:
     name: puppet-openstack-libraries-puppet-beaker-rspec-ubuntu-bionic