add a feature to balance write to load lowest rw disk

[yogoloth]

I add a feature to balance write to load lowest rw disk.
I use this to bind a 20T disk over several Huawei cloud server‘s disks for backup. and write 100G perday , I think it's stability and hope it will be helpful for somebody .

to enable this feather by supply -o lbwrite when mount.
and do a little modify in mount.unionfs ,now it can work for me.
I add this fstab example in project, too.

[rpodgorny]

hi. thanks for the patch! unfortunately, i can't merge it as-is because:

1. we don't do tests in bash anymore, please migrate to python tests
2. try to add a proper test (in python)
3. clean up the commits a bit (ammend? rebase?)

ideally, i would like to see just two commits:

1. add the functionality
2. add the tests

don't worry about the english and terminology, i can fix that later.