Terraform: Upgrade to 0.6.15

This isn't as big a jump as this individual commit suggests because we were
already using a pre-release version of 0.6.15 in 57cc1a9. We are upgrading
so that we no longer need to build from source to benefit from these fixes:

- hashicorp/terraform#5774
- hashicorp/terraform#5904

The CHANGELOG doesn't mention any backwards-incompatible changes. New
checksum has been taken from:

- https://releases.hashicorp.com/terraform/0.6.15/terraform_0.6.15_SHA256SUMS

terraform/Dockerfile

@@ -1,8 +1,8 @@
 FROM alpine:3.3
 
 ENV PATH $PATH:/usr/local/bin
-ENV TERRAFORM_VER 0.6.13
-ENV TERRAFORM_SUM 8eeddca53958d5c871de93a624eb0f5971f97cbaf107e0b2d1cc289f2ac21b79
+ENV TERRAFORM_VER 0.6.15
+ENV TERRAFORM_SUM 2a81faa54ed6c5e7c065444617fc999f0ab6d433e4e03a0ad599892e74ffff6b
 ENV TERRAFORM_ZIP terraform_${TERRAFORM_VER}_linux_amd64.zip
 ENV BINARY_WHITELIST \
   terraform \