Enable TLS by default if /ssl directory is present

Generates needed keys and certs.
If only one element in the key/cert pair is present, nothing is
overriden; instead, the user is asked to either remove the existing
element, or put the missing one back.

Uses TLSv1, since TLSv1.1 nor TLSv1.2 are available in the current
version of python 2.7.

Usage: docker run -d -p 5000:5000 -v /etc/docker/certs.d:/ssl registry

There are no breaking changes, since the /ssl directory is not present
by default.

Signed-off-by: Tibor Vass <teabee89@gmail.com>
Sorry to go back and worth on this, but after taking a closer look I'm not that refers to the same thing.

REGISTRY_HOST refer to the hostname within the container, i.e: 0.0.0.0 means listen on the container IP.

Here we want to refer to the container as it is addressed from the host, when publishing its port: i.e boot2docker ip.