servicedisco: ensure service uniqueness in job validation

[shoenig]

87ef517: some cleanup to use set in the validateServices function

0cc18331058f7ff8d14907f2f71d0870d4515e87: track uniqueness of services in validateServices function

Fixes #13814

Closes #13836 - This PR went in the other direction - to support groups with services that had the same <name, task|group, port_label>. The change is gnarly - basically we would need to add something like a hash of the tags of a service to the unique ID generated by MakeAllocationServiceID - which is then used as an index in the state store. All services with tags would be given new IDs, causing unwanted churn in large clusters. The benefit doesn't come close to justifying the means, so I'm going to abandon the feature and just validate the intrinsic invariant we already have (see: this PR).

Spot check:

example validation on identical services with port label set

Error submitting job: Unexpected response code: 500 (1 error occurred:
	* Task group database validation failed: 1 error occurred:
	* Task group service validation failed: 1 error occurred:
	* Services are not unique: [group->db:p1]

same but without port label set

Error submitting job: Unexpected response code: 500 (1 error occurred:
	* Task group database validation failed: 1 error occurred:
	* Task group service validation failed: 1 error occurred:
	* Services are not unique: [group->db]

job file

job "simple" {
  datacenters = ["dc1"]
  type        = "service"

  group "database" {

    service {
      name     = "db"
      tags     = ["two"]
      # port = "p1"
      provider = "nomad"
    }

    service {
      name     = "db"
      tags     = ["one"]
      # port = "p1"
      provider = "nomad"
    }

    task "db" {
      driver = "raw_exec"
      config {
        command = "bash"
        args    = ["-c", "sleep 15000"]
      }
    }
  }
}

[jrasell]

LGTM!

[github-actions]

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