WIP: Feature gate emptyDir.sizeLimit

Add a configurable gate around the emptyDir.sizeLimit property, as it is alpha from Kubernets v1.7.x, but seems more accessible from v1.8.x onward.

Also see https://github.com/kubernetes/kubernetes/issues/43607

Relates to #119 (closed)

Discussion to be had, marking WIP so that it is not accidentally merged.