We require that each "credentials" block has a valid hostname and that
there be no more than one "credentials_helper" block.
There are some more sophisticated validations we could do here, such as
checking if the same host is declared more than once, but since this
config handling will be rewritten to use HCL2 in the near future, and this
sort of check is easier to do in the HCL2 API, we just check the basic
stuff for now and plan to revisit later.
This allows the use case where installation of a plugin can simply say
to add `$GOPATH/bin/foo` to your terraformrc and the user can do that
verbatim. Additionally terraformrc files become portable for certain
environments which are self-contained.
I was hesitant at first about this because it diverges the syntax a bit
from our standard interpolation syntax. However, due to the special
nature of terraformrc and the strong use cases cited I'm okay with this.
Martin Atkins