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.