Allow to customize image retention time

[kelson42]

We have currently two different retention times:

• standard image have a short retention time, currently set at 14 days
• Auto images wich have a retention of around a month (the time to renew the image and provide that way always a recent version via stable redirect)

But if for some reason a user needs it longer then we have no easy way to change an image rentention time, neither at configuration time nor a posteriori.

This scenario is not really necessary if the creator of an image is the consumer (the one writting an SD card) because 14 days are normally enough to download the image. This should be always the case in a B2C scenario.

In a B2C scenario it can be the same but it can be as well more conplex. Many people can easily be involved and it might be needed for many different people to download the same image and the propbabky over a time span bigger than 14 days.

[kelson42]

This was at least the case once for Anthere with Wikifundi images,