lifecycle: successful prestart tasks should not fail deployments

In 492d62d we prevented poststop tasks from contributing to allocation health
status, which fixed a bug where poststop tasks would prevent a deployment from
ever being marked successful. The patch introduced a regression where prestart
tasks that complete are causing the allocation to be marked unhealthy. This
changeset restores the previous behavior for prestart tasks.

client/allochealth/tracker.go

@@ -285,7 +285,7 @@ func (t *Tracker) watchTaskEvents() {
 			}
 
 			// One of the tasks has failed so we can exit watching
-			if state.Failed || !state.FinishedAt.IsZero() {
+			if state.Failed || (!state.FinishedAt.IsZero() && t.lifecycleTasks[taskName] != structs.TaskLifecycleHookPrestart) {
 				t.setTaskHealth(false, true)
 				return
 			}