Allow failure on tf-elax_ubuntu18-calico (kubernetes-sigs#7814)

tf-elax_ubuntu18-calico is so flake today. The test job is failed
due to SSH connectivity check error after deploying virtual machines
which are used for Kubernetes nodes.
This allows failure on the job to see the test situation without
pull request merger failures.

.gitlab-ci/terraform.yml

@@ -207,6 +207,7 @@ tf-elastx_ubuntu18-calico:
   extends: .terraform_apply
   stage: deploy-part3
   when: on_success
+  allow_failure: true
   variables:
     <<: *elastx_variables
     TF_VERSION: $TERRAFORM_14_VERSION