client: respect client_auto_join after connection loss

[tgross]

Fixes #11404

The consul.client_auto_join configuration block tells the Nomad
client whether to use Consul service discovery to find Nomad
servers. By default it is set to true, but contrary to the
documentation it was only respected during the initial client
registration. If a client missed a heartbeat, failed a
Node.UpdateStatus RPC, or if there was no Nomad leader, the client
would fallback to Consul even if client_auto_join was set to
false. This changeset returns early from the client's trigger for
Consul discovery if the client_auto_join field is set to false.

---

To test this I ran a client node and a server node with Consul under Vagrant. Then I blocked access to the server's ports and Consul:

$ sudo iptables -A INPUT -p tcp --destination-port 4647 -j DROP
$ sudo iptables -A INPUT -p tcp --destination-port 8500 -j DROP

Then waited for the client to log that it had lost it's connection to the server, and removed the server rule while leaving the Consul rule in place:

$ sudo iptables --line-numbers -L INPUT

Chain INPUT (policy ACCEPT)
num  target     prot opt source               destination
1    DROP       tcp  --  anywhere             anywhere             tcp dpt:8500
2    DROP       tcp  --  anywhere             anywhere             tcp dpt:4647

$ sudo iptables -D INPUT 2

And after a few moments, the client restored its connection with the server without needing to have Consul, as expected. I also did the same exercise without dropping packets to Consul and watched the logs to ensure that no requests were being made from the client to Consul to get the server address.

[tgross]

cc @davemay99 as a heads up, as I think I recall seeing this kind of thing in a support issue as well.

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