Don't specify priority in fstab if -1 (default) is used

For some reason, such fstab line doesn't work and it's better not to rely on -1
meaning the default forever.

Related: rhbz#1202505

blivet/formats/swap.py

@@ -114,7 +114,7 @@ def _getPriority(self):
 
     def _getOptions(self):
         opts = ""
-        if self.priority is not None:
+        if self.priority is not None and self.priority != -1:
             opts += "pri=%d" % self.priority
 
         return opts