Add configuration defaults to allow minikube support

[seryl]

Add automatic TLS support if DOCKER_CERT_PATH is configured and support DOCKER_HOST.

DOCKER_URL seems to be a deprecated or library-specific variable; I've kept support for it but preferred DOCKER_HOST if it exists.

minikube:

> minikube docker-env
export DOCKER_TLS_VERIFY="1"
export DOCKER_HOST="tcp://192.168.99.100:2376"
export DOCKER_CERT_PATH="/Users/jtoft/.minikube/certs"
export DOCKER_API_VERSION="1.23"
# Run this command to configure your shell:
# eval $(minikube docker-env)

docker-machine:

https://docs.docker.com/machine/reference/env/

[aeneasr]

Nice!