installer: update tf-fmt to use latest tf version and mark it optional

Installer [1] is in progress to move to terraform version 0.12, which has breaking changes for formatting.
To add to that, `upi` terraform templates will be moved over to 0.12 at a later stage.

Therefore marking this optional for the time being until we have moved entire code base to 0.12.

[1]: openshift/installer#1739

ci-operator/jobs/openshift/installer/openshift-installer-master-presubmits.yaml

@@ -1054,6 +1054,7 @@ presubmits:
     context: ci/prow/tf-fmt
     decorate: true
     name: pull-ci-openshift-installer-tf-fmt
+    optional: true
     rerun_command: /test tf-fmt
     spec:
       containers:
@@ -1064,7 +1065,7 @@ presubmits:
         env:
         - name: IS_CONTAINER
           value: "TRUE"
-        image: quay.io/coreos/terraform-alpine:v0.11.8
+        image: quay.io/coreos/terraform-alpine:v0.12.0-rc1
         imagePullPolicy: Always
         name: ""
         resources: