Add SSL services annotation

[jmastr]

If an application in a container requires HTTPS, one needs to set the
'proxy_path https://...' instead of 'proxy_pass http://...'. With this
patch one can use the annotation:

'nginx.org/ssl-services: "service1[,service2,...]"'

to specify, which services require HTTPS.

Fixes #61

[pleshakov]

Hi @jmastr
Thanks for the pull request! Looks good!

I have a few small suggestions:

1. Could you rename the annotation from nginx.org/ssl-backends to nginx.org/ssl-services? This will make the naming consistent: we already have nginx.org/websocket-services and nginx.com/sticky-cookie-services. My bad suggesting the wrong name in proxy_pass hard-coded to HTTP #61
2. As a consequence of renaming, please rename the example folder to ssl-services, the title of the README.md to SSL Services Support, the name of the service from ssl-backend-svc to ssl-svc.
3. Could you also replace an application speaking HTTPS with an application that requires HTTPS , one of which is speaking SSL with one of which requires HTTPS and is a service for the SSL backend application with is a service for an HTTPS application

[jmastr]

@pleshakov done