Introduce environment variable OTOBO_NGINX_SSL_DIR

[bschmalhofer]

For setting up the HTTPS terminating proxy nginx, user must currently configure the complete path to the certificate file in the environment settings OTOBO_NGINX_SSL_CERTIFICATE and OTOBO_NGINX_SSL_CERTIFICATE_KEY. This is even required when they use the same file and directory names that are mentioned in the installation manual. The setup is also error prone, as it easy to forget about specifying the complete path. So this can be improved. The requirement is that existing configs,with the complete path, continue to work as expected.

TODO:

• remove unhelpful default that is set up in the image itself
• provide sane defaults, as laid out in the installation manual, in docker-compose/override-https*.yml
• support for OTOBO_NGINX_SSL_DIR
• compatability with the current behavior
• update the installation manual

[bschmalhofer]

A related issue is #75.

[bschmalhofer]

Another related issue is #81.