Add 'none' shell for docker-env output

[r2d4]

This is for consumers who want the key/value output, but don't need the
shell directives.

Sample output

$ minikube docker-env --shell none
DOCKER_TLS_VERIFY=1
DOCKER_HOST=tcp://192.168.39.124:2376
DOCKER_CERT_PATH=/home/mrick/.minikube/certs
DOCKER_API_VERSION=1.23

$ minikube docker-env --shell none -u
DOCKER_TLS_VERIFY=
DOCKER_HOST=
DOCKER_CERT_PATH=
DOCKER_API_VERSION=

cc @loosebazooka

[codecov-io]

Codecov Report

Merging #1792 into master will decrease coverage by 0.11%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1792      +/-   ##
==========================================
- Coverage   37.27%   37.16%   -0.12%     
==========================================
  Files          51       51              
  Lines        3367     3377      +10     
==========================================
  Hits         1255     1255              
- Misses       1929     1939      +10     
  Partials      183      183

Impacted Files	Coverage Δ
cmd/minikube/cmd/env.go	56.8% <0%> (-3.58%)	⬇️

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[loosebazooka]

🍾