coreos-install: Guard against sporadic blockdev failures

The blockdev command may sporadically fail with "BLKRRPART: Device or
resource busy" due to what appears to be interaction with udev behind
the scenes. The following point to further information:

https://groups.google.com/forum/#!topic/coreos-user/wFoFr-TyQ6s (another coreos user)
https://rwmj.wordpress.com/2012/01/19/udev-unexpectedness/ (directly about this issue)

Here is actual runtime evidence of it happening and then doing the right
thing with a retry:

+ BLOCKDEV_REREAD_MAX=4
+ (( m=BLOCKDEV_REREAD_MAX ))
+ (( m-- ))
+ blockdev --rereadpt /dev/sda
+ sleep 1
+ (( 1 ))
+ (( m-- ))
+ blockdev --rereadpt /dev/sda
+ break
+ [[ 2 -le 0 ]]