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.

This is backwards. TLS verification should be default.  Users should have to explicitly disable it. Remember that the TLS verification is only happening if we talk to the TCP socket. We shouldn't be silently insecure and vocally secure -- instead, we should be vocally insecure and silently secure.