Add 'nomad.advertise.address' to client meta via NomadFingerPrint

[jrasell]

This commit adds the Nomad local client advertise address in the form host:port to the client attributes. This is helpful in situation where the task is making Nomad API calls as the current alternative is to use hardcoded IPs, DNS LB names or create dummy ports and use the env vars resulting from this.

Once this is merged, I will raise a PR for doc updates to reflect the change.

Tested locally with job:

driver = "raw_exec"
      config {
        command = "echo"
        args = ["${attr.nomad.advertise.address}"]
      }

Result:

$ nomad logs 1e4148e2
127.0.0.1:4646

Nomad status output

$ nomad node status -verbose 0817b9ac |grep 'nomad.advertise.address'
nomad.advertise.address       = 127.0.0.1:4646

Tests pass locally:

$ /usr/local/bin/go test -timeout 30s github.com/hashicorp/nomad/client/fingerprint -run ^TestNomadFingerprint$

ok  	github.com/hashicorp/nomad/client/fingerprint
Success: Tests passed.

Closes #4381

[dadgar]

@jrasell I would be more a fan of adding it to the client's meta via the nomad_fingerprinter and then tasks that need it can add it. Want to avoid infinite number of builtin env vars being exported.

[jrasell]

@dadgar makes perfect sense; i'll work on updating to make the change in that manner.

[jrasell]

@dadgar updated as per your advice

[preetapan]

@jrasell the most recent update looks good but there is a merge conflict, could you rebase your branch against master?

[jrasell]

@preetapan merge conflict has been resolved

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