Disable Transparent Huge Pages (THP) on Redis machines

I don't think we've had to do this before, and I don't see warnings in the Redis logs but Redis strongly recommends disabling THP to avoid latency:

• https://redis.io/topics/latency
• https://github.com/antirez/redis/issues/3176
• http://antirez.com/news/84

It can be disabled simply by:

echo never > /sys/kernel/mm/transparent_hugepage/enabled

Right now it's set as the following:

# cat /sys/kernel/mm/transparent_hugepage/enabled 
always madvise [never]

/cc: @jarv, @andrewn