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.

@crosbymichael: fixed.

@ewindisch: Agreed, but that's a longer-term goal - see #8265. The purpose of this change is in fact to make boot2docker 1.3 secure-by-default. The plan:

1. docker 1.3 supports `DOCKER_TLS_VERIFY`.
2. boot2docker generates certs and require TLS.
3. `boot2docker up` downloads those certs and prints out the correct instructions for setting `DOCKER_HOST`, `DOCKER_CERT_PATH` and `DOCKER_TLS_VERIFY`.