This PR switches the Nomad repository from using govendor to Go modules
for managing dependencies. Aspects of the Nomad workflow remain pretty
much the same. The usual Makefile targets should continue to work as
they always did. The API submodule simply defers to the parent Nomad
version on the repository, keeping the semantics of API versioning that
currently exists.
Update github.com/aws/aws-sdk-go and github.com/hashicorp/go-discover to
pick up support for EC2 Metadata Instance Service v2 changes.
Follow up to https://github.com/hashicorp/go-discover/pull/128 .
