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.
* client/metrics: modified metrics to use (updated) client copy of allocation instead of (unupdated) server copy
* updated armon/go-metrics to address race condition in DisplayMetrics
