Integrate with Nomad workload identiy

[lgfa29]

Nomad 1.4.0 introduced the concept of workload identities, which are used to assign a verifiable identification to allocations running in Nomad. This can be used to greatly simplify ACL policy management.

Currently this feature is only supported when using Nomad Variables, but will likely be expanded in the future to the point where the Autoscaler can make use of it.

[krarey]

+1 for this, it's a perfect use case for the Task API but it seems like the autoscaler's client library needs updating to support UNIX domain sockets.

[lgfa29]

We need to update Nomad's api package and then pull and configure whatever necessary to make use of the task socket. But yes this should be, hopefully, a quick process once github.com/hashicorp/nomad/api is updated 🙂