client: respect client_auto_join after connection loss (#11585)

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`.

.changelog/11585.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+client: Fixed a bug where clients would ignore the `client_auto_join` setting after losing connection with the servers, causing them to incorrectly fallback to Consul discovery if it was set to `false`.
+```

client/client.go

@@ -2690,11 +2690,13 @@ func taskIsPresent(taskName string, tasks []*structs.Task) bool {
 
 // triggerDiscovery causes a Consul discovery to begin (if one hasn't already)
 func (c *Client) triggerDiscovery() {
-	select {
-	case c.triggerDiscoveryCh <- struct{}{}:
-		// Discovery goroutine was released to execute
-	default:
-		// Discovery goroutine was already running
+	if c.configCopy.ConsulConfig.ClientAutoJoin != nil && *c.configCopy.ConsulConfig.ClientAutoJoin {
+		select {
+		case c.triggerDiscoveryCh <- struct{}{}:
+			// Discovery goroutine was released to execute
+		default:
+			// Discovery goroutine was already running
+		}
 	}
 }