Bump Terraform from 0.11.7 to 0.11.8

0.11.8 was cut on 2018-08-15 [1,2].

The image bump moved the terraform binary into the container's PATH
with sallyom/scripts-images@931c8ee8 (terraform-alpine: Install
terraform into $PATH, 2018-08-16, sallyom/scripts-images#3), but the
installer's tf-fmt.sh was updated to match in
openshift/installer@00323486 (Bump Terraform from 0.11.7 to 0.11.8,
2018-06-09, openshift/installer#218).  Bumping the image here depends
on using that bumped version of tf-fmt.sh.

[1]: https://github.com/hashicorp/terraform/releases/tag/v0.11.8
[2]: https://releases.hashicorp.com/terraform/0.11.8/

Author: wking

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

@@ -278,7 +278,7 @@ presubmits:
         env:
         - name: IS_CONTAINER
           value: "TRUE"
-        image: quay.io/coreos/terraform-alpine:v0.11.7
+        image: quay.io/coreos/terraform-alpine:v0.11.8
         name: ""
         resources: {}
     trigger: ((?m)^/test( all| tf-fmt),?(\s+|$))