Only warn about 'driver' not being set if the priority list is unset

Currently we would display an error when the user does not specify a `driver` in
their config file. This has been present for historical reasons mostly to
prevent users from accidentally getting the vfs
driver (#1460 (comment)). Now
that most systems support the overlay driver natively, we can reduce this to a
warning and only warn about it if the driver_priority list is unset. If it is
provided, then clearly the user or the distribution wanted for c/storage to pick
a driver itself and the warning would be only confusing to users.

Signed-off-by: Dan Čermák <dcermak@suse.com>

types/options.go

@@ -387,8 +387,8 @@ func ReloadConfigurationFile(configFile string, storeOptions *StoreOptions) erro
 		logrus.Warnf("Switching default driver from overlay2 to the equivalent overlay driver")
 		storeOptions.GraphDriverName = overlayDriver
 	}
-	if storeOptions.GraphDriverName == "" {
-		logrus.Errorf("The storage 'driver' option must be set in %s to guarantee proper operation", configFile)
+	if storeOptions.GraphDriverName == "" && len(storeOptions.GraphDriverPriority) == 0 {
+		logrus.Warnf("The storage 'driver' option should be set in %s. A driver was picked automatically.", configFile)
 	}
 	storeOptions.GraphDriverPriority = config.Storage.DriverPriority
 	if config.Storage.RunRoot != "" {