Change tf-elax_ubuntu18-calico to manual job

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 changes the job to manual job to skip it on normal pipeline.

.gitlab-ci/terraform.yml

@@ -206,7 +206,7 @@ tf-elastx_cleanup:
 tf-elastx_ubuntu18-calico:
   extends: .terraform_apply
   stage: deploy-part3
-  when: on_success
+  when: manual
   variables:
     <<: *elastx_variables
     TF_VERSION: $TERRAFORM_14_VERSION