Simplify the whole code

[moufmouf]

Hey @BertrandGouny !

I'm using your image on a regular basis and today, and first, I would like to thank you for your work. I encountered an issue today. I wanted to be able to configure MYSQL backups and S3 uploads.
Those features are available in "backup-manager" but you don't expose the environment variables "BACKUP_MANAGER_XXX" in your Docker image.

I started writing a PR to add support for those variables. I started editing the startup.sh file to add all environment variables.

https://github.com/osixia/docker-backup-manager/blob/stable/image/service/backup-manager/startup.sh#L17-L27

And then it hit me.
I've had a look at the code of backup-manager itself.
It is actually a simple collection of shell scripts.

And look: https://github.com/dkogan/Backup-Manager/blob/master/backup-manager#L200
The configuration file as actually a shell script, and all lines in the configuration file are actually environment variables.

So instead of trying to build a configuration file at startup, it would be way easier to simply delete the configuration file and document that you can use any configuration options of backup-manager as an environment variable passed in Docker.

Actually, this is not 100% true, you will face issues with the environment variables types as arrays (that are not supported by Docker). But you could definitely shrinken your configuration file by only keeping these lines: https://github.com/osixia/docker-backup-manager/blob/stable/image/service/backup-manager/assets/backup-manager.conf#L281-L316

Everything else can be removed.

Benefits:

• less code
• so less bugs
• easier migration to new versions of backup-manager
• more features!

What do you think?