Allow to max out the drive rather than guessing the max size

[nazar-pc]

Space Acres allows to specify the size for the farm, but there are a few UX issues:

• if you specify the size that is larger than available space on disk, it will fail to start and there is no nice recovery from this situation right now (see Re-configuration support #16)
• it takes a few attempts to actually guess the max size without exceeding disk size

As such it would be nice to support an option to fill the whole drive and let the app figure out how big the size should be for that to happen.

The strategy could be pretty simple:

• start with the farm size that is equal to size of the drive -1GiB
• if that fails, go down 1GiB at a time until it farm allocation succeeds

In config we can store None until farm size is known, after which it will be replaced with a fixed value identified during farm startup.