Add automatic Consul constraint for Nomad jobs that depend on Consul

[mikenomitch]

Proposal

When a Nomad job declares a service stanza, it relies on Consul to function properly. This should create a constraint on thee job automatically to only run on client nodes that have Consul running. It is possible to do this currently, as Consul is fingerprinted, but many people will not add the constraint manually.

Use-cases

This will help avoid placement of jobs onto client nodes that do not have Consul running, and will make it less likely that tasks cannot run properly.

[github-actions]

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