Add support for extending image and enabling SSL connections

[hhorak]

The image should be generally flexible, which means it should be easy to extend the functionality of the image by end users. A common thing that the end users need to change is providing own configuration file or setting up secured connection over SSL.

In some cases, users prefer to extend the image by creating a thin layer on top of the provided image and have the own bits available in the image itself. However, it is also common to use kubernetes secrets to extend the image functionality, by providing a configuration file or pair of SSL certificates.

In some cases, making the image flexible required to refactor the image a bit more; there are PRs for other images, that already support these features below, they could serve for inspiration. An important part of the changes is also documentation how the extending works, what paths should be used, etc.

nginx: sclorg/nginx-container#41 (especially interesting from varnish PoV, since the changes might be quite similar)
httpd: sclorg/httpd-container#28
php: sclorg/s2i-php-container#183
mariadb: sclorg/mariadb-container#45
mysql: sclorg/mysql-container#201
postgresql: sclorg/postgresql-container#208
mongodb: sclorg/mongodb-container#239

[phracek]

@pkubat Does it make sense to be implemented in case varnish is not running in OpenShift environment? If not, please close it.