Update start and readme

[fpagnoux]

• Use mes-aides.ini confing file when running start.sh
• Update REAMDE.md

It seems that only mes-aides uses this repository, so let's be honest and not pay genericity cost for nothing. The goal of this repository is then just to help to deploy an openfisca instance compatible with mes-aides, and start.sh doesn't need a parameter.

[MattiSG]

Well, the whole point of this repo was to ease the deployment of a production OpenFisca instance. Has your work with OpenFisca packaging made it redundant (i.e. can one now just pip install openfica)?
If yes, then let's reassess the importance of this repo as a whole.
If no, then I'd rather keep it generic. The fact that we right now are the only users doesn't make the need go away. Plus, I'd argue the genericity costs have already been paid, and passing a param is not that expensive.

[MattiSG]

The adaptation to Mes Aides has been made in betagouv/aides-jeunes#384, without the need to make this repo less generic, thanks to proper packaging of OpenFisca.