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.
Fixes#7854
Nomad requires a version of go-getter that is currently in PR (https://github.com/hashicorp/go-getter/pull/256)
We also require some recent bug fix to go-getter around the handling of URL redirects.
Update our vendor'd copy of go-getter to the newly rebased umask changes so that we can incorporate
the latest changes for go-getter.
