Race condition when executing terraform binary

[dansimau]

When Terraform is not installed, it is automatically downloaded.

Sometimes in the tests we are seeing:

	Error:      	Should be empty, but was failed to load YAML from file: astro.yaml; unable to detect Terraform version: fork/exec /home/travis/.tvm/linux/amd64/0.8.8/terraform: text file busy

This probably happens when two threads try to download Terraform at the same time and one tries to execute it while the other one is writing to it.

See extensive discussions on this:
golang/go#22220
golang/go#3001