Replace xfs with ext4 for r3 SSD instances

In my experience xfs had a lot of problems during shuffle heavy workloads. All the problems I observed went away when I replaced the file system with ext4. We might be able to tune xfs to be better; however, ext4 just provides better out of the box behavior here without us having to dig deeper into file system tuning. 
Yes - it is not obvious whether we should turn TRIM support on by default. I added another line of comment to explain that.