cli: correctly use and validate job with vault token set

This PR fixes `job validate` to respect '-vault-token', '$VAULT_TOKEN',
'-vault-namespace' if set.

command/testdata/example-vault.nomad

@@ -0,0 +1,14 @@
+job "vault" {
+  datacenters = ["dc1"]
+  group "group" {
+    task "task" {
+      driver = "docker"
+      config {
+        image = "alpine:latest"
+      }
+      vault {
+        policies = ["my-policy"]
+      }
+    }
+  }
+}