Add system-config to rspec-infra jobs

The legacy-puppet-beaker-rspec-infra and
legacy-puppet-beaker-rspec-centos-7-infra jobs appear to be failing
due to a missing system-config repository

Change-Id: I9eb17646bd949c3bd84fa61f92881e9fac015de5

zuul.d/zuul-legacy-jobs.yaml

@@ -3235,6 +3235,7 @@
     parent: legacy-puppet-beaker-rspec
     required-projects:
       - openstack-infra/project-config
+      - openstack-infra/system-config
       - openstack-infra/puppet-bugdaystats
       - openstack-infra/puppet-mysql_backup
       - openstack-infra/puppet-openstackci