Automatic merge from submit-queue
Fix AWS device allocator to only use valid device names
According to
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/device_naming.html
we can only use /dev/xvd[b-c][a-z] as device names - so we can only
allocate upto 52 ebs volumes on a node.
fixes#41453
cc @justinsb @kubernetes/sig-storage-pr-reviews