Offer to insert specific configuration variables

[kaiyou]

Daemons embedded in Freeposte have many configuration options, most of which are not tunable through the usual Freeposte interface. It would be interesting to be able to tune them using specific freeposte.env entries or configuration files.

[kaiyou]

As suggested by @jkarlobs in #26 :

Yes, the ability to set specific postfix options from the main freeposte.env file I think that is a great idea, even including a config folder with dovecot config files, postfix config files (main.cf and eventual lookup tables file for some entries) , etc. and when recreating the containers, this entries of these config files override the entries of main configuration. If this files are empty then we do nothing.

[kaiyou]

PR !30 implements this for Postix and provides a clear framework for other implementations. I will try and implement this for Dovecot, maybe using inclusions because the Dovecot configuration is not as linear as Postfix (cannot read line by line).