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.

This adopts pattern used by Vault, where we split CircleCI yaml config
into multiple files that get packed and translated to 2.0.

This has two motivations: First, to ease translating config to CircleCI
2.0 so it can run on Enterprise private repository.  Second and most
importantly, it also adding Enterprise specific jobs in separate files
with reduced config file merging conflict resolution.