Upgrade our consul/api import to the equivelent of consul@v1.8.1 which includes
a bug fix necessary for #6913. If consul would publish a proper api/ submodule tag
we could reference that.
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.
