add network proxy support to k8s 1.19

[bcressey]

Issue number:
N/A

Description of changes:
Adds the missing EnvironmentFile line to kubelet.service for k8s-1.19.

Testing done:
Confirmed that all the kubelet.service files match with the exception of 1.19:

❯ sha512sum packages/kubernetes-1.1*/kubelet.service
9f10ef80a617803a35937d86047d2557833c1d009165dd4609947574670c0f2004ecfa2625002582585623b0e53b47fdd16a263214017a9f392ab0780f802ad6  packages/kubernetes-1.15/kubelet.service
9f10ef80a617803a35937d86047d2557833c1d009165dd4609947574670c0f2004ecfa2625002582585623b0e53b47fdd16a263214017a9f392ab0780f802ad6  packages/kubernetes-1.16/kubelet.service
9f10ef80a617803a35937d86047d2557833c1d009165dd4609947574670c0f2004ecfa2625002582585623b0e53b47fdd16a263214017a9f392ab0780f802ad6  packages/kubernetes-1.17/kubelet.service
9f10ef80a617803a35937d86047d2557833c1d009165dd4609947574670c0f2004ecfa2625002582585623b0e53b47fdd16a263214017a9f392ab0780f802ad6  packages/kubernetes-1.18/kubelet.service
37f3e8b50267fac4a64bf1af2d7ab0baa2ab77ad2c5936f26f59aace1e7b94730598823293b8a9158d29d9a3470b21379d739ce3d378cdeb8e4402ee97c96d5c  packages/kubernetes-1.19/kubelet.service

The remaining difference is the volume-plugin-dir argument which was moved to the kubelet config file.

❯ diff packages/kubernetes-1.18/kubelet.service packages/kubernetes-1.19/kubelet.service
29d28
<     --volume-plugin-dir /var/lib/kubelet/plugins/volume/exec \

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.