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>
wonder if we should keep the `ENTRYPOINT` in python and push the extra logic about in-place cert generate to `generate_certs`