fingerprint: add support for fingerprinting multiple Consul clusters
Add fingerprinting we'll need to accept multiple Consul clusters in upcoming
Nomad Enterprise features. The fingerprinter will create a map of Consul clients
by cluster name. In Nomad CE, all but the default cluster will be ignored and
there will be no visible behavior change.
Ref: https://github.com/hashicorp/team-nomad/issues/404