Remove the now-unused original configuration blocks for Consul and Vault from
the server. When the server needs to refer to a Consul or Vault block it will
always be for a specific cluster for the task/service. Add a helper for
accessing the default clusters (for the servers own use).
This is one of three changesets for this work. The remainder will implement the
same changes in the `client` package and on the `command/agent` package.
As part of this work I discovered that the job submission hook for Vault only
checks the enabled flag on the default cluster, rather than the clusters that
are used by the job being submitted. This will return an error on job
registration saying that Vault is disabled. Fix that to check only the
cluster(s) used by the job.
Ref: https://github.com/hashicorp/nomad/issues/18947
Fixes: https://github.com/hashicorp/nomad/issues/18990
1ef99f0536