Make vagrant-ubuntu20-flannel voting (#9469)

We made all vagrant jobs non-voting because those jobs were not stable.
However the setting allowed a pull request which broke vagrant jobs
completely merged into the master branch.
To avoid such situation, this makes one of vagrant jobs voting.
Let's see the stability of the job.

.gitlab-ci/vagrant.yml

@@ -43,6 +43,7 @@ vagrant_ubuntu20-flannel:
   stage: deploy-part2
   extends: .vagrant
   when: on_success
+  allow_failure: false
 
 vagrant_ubuntu16-kube-router-sep:
   stage: deploy-part2