oauth: Allow auth_url and token_url to be set in the config file

If set in the config file, these override the ones configured into the
remote.  This enables alternative oauth servers to be used for all
oauth remotes.  This can only be altered by editing the config file
for the moment.

oauthutil/oauthutil.go

@@ -209,6 +209,16 @@ func overrideCredentials(name string, origConfig *oauth2.Config) (config *oauth2
 		config.ClientSecret = ClientSecret
 		changed = true
 	}
+	AuthURL := fs.ConfigFileGet(name, fs.ConfigAuthURL)
+	if AuthURL != "" {
+		config.Endpoint.AuthURL = AuthURL
+		changed = true
+	}
+	TokenURL := fs.ConfigFileGet(name, fs.ConfigTokenURL)
+	if TokenURL != "" {
+		config.Endpoint.TokenURL = TokenURL
+		changed = true
+	}
 	return config, changed
 }