Add the ability to add a dynamic config file via variable

[mleutenegger]

Currently, all dynamic config is pulled from docker. A user could use the config options to add any other provieder, however the most prominent usecase would be to add a file provider. In order to simplify this process, we should allow users to add a custom config file via yaml config.

The following features should be implemented:

• there is a config option trraefik_dynamic_file_config
• when trraefik_dynamic_file_config is set, a file is rendered in the traefik dir
• when trraefik_dynamic_file_config is set, a file provider is added to the config