Use rs.slaveOk to fix mongo 3.6.13 backup (#1883)

Signed-off-by: Arnob Kumar Saha <arnob@appscode.com>

Author: ArnobKumarSaha

pkg/backup.go

@@ -701,7 +701,7 @@ func lockConfigServer(configSVRDSN, secondaryHost string) error {
 			"config",
 			"--host", secondaryHost,
 			"--quiet",
-			"--eval", "rs.secondaryOk(); db.BackupControl.find({ '_id' : 'BackupControlDocument' }).readConcern('majority');",
+			"--eval", "rs.slaveOk(); db.BackupControl.find({ '_id' : 'BackupControlDocument' }).readConcern('majority');",
 		}, mongoCreds...)
 
 		if err := sh.Command(MongoCMD, args...).UnmarshalJSON(&v); err != nil {