stop emulating envFrom and start using it directly

[kadel]

Right now we are doing some magic to allow envFrom to work for k8s versions <=1.5 (we are expanding it , which is quite limited in what it can do compare to native envFrom)

1.5 is getting older, and I think it might be time to drop support for this.
If users want to use kedge with 1.5 and below they still can, they just can't use envFrom as they can't use it when writing Kubernetes artifacts.

related to discussion in #141