command, docs: create and document consul token configuration for connect acls (gh-6716)

This change provides an initial pass at setting up the configuration necessary to
enable use of Connect with Consul ACLs. Operators will be able to pass in a Consul
Token through `-consul-token` or `$CONSUL_TOKEN` in the `job run` and `job revert`
commands (similar to Vault tokens).

These values are not actually used yet in this changeset.

command/agent/testdata/basic.hcl

@@ -174,6 +174,7 @@ consul {
   client_service_name    = "nomad-client"
   client_http_check_name = "nomad-client-http-health-check"
   address                = "127.0.0.1:9500"
+  allow_unauthenticated  = true
   token                  = "token1"
   auth                   = "username:pass"
   ssl                    = true