Add DOCKER_TLS_VERIFY environment variable, equivalent to --tlsverify flag

This makes it possible to make the Docker client "secure by default"
without wrapping the binary in a shell alias so that `--tlsverify` is
always passed.

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>
Merge pull request #8503 from aanand/tlsverify-environment-variable

Add DOCKER_TLS_VERIFY environment variable, equivalent to --tlsverify flag