Add Host to PodSpec and add a predicate to make the scheduler work.

@bgrant0607 would this be a good way to format newly attached GCEPDs/EBS volumes?  In #2609 I suggested having kubelet format a newly attached disk before mounting the filesystem.  I can see a per-node controller performing this action instead.  It could effectively throttle the number of concurrent formatting operations and prevent the host from getting crushed.

Your feedback and advice on #2609 would be invaluable :)

Setting the host in Spec would also easily accommodate the hostDir example laid out in #1515.