Add configuration flag allowing the use of a custom b2d ISO source URL

Until now we have been using a heavily patched fork of the now-retired bash-based boot2docker script, but all we need on the script-side (apart from the ability to use a custom ISO) is now in the go port, so I'd like to move on and benefit from the all-in-one DMG installer on OS X.

I think there will always be a need for custom ISOs. We am using one currently in order to a company proxy for the docker engine and auto-create a larger swap partition at the first boot. These are of course things that could be made configurable, but is it really worth it? There will be always specific/exotic needs adding up over time, so this allows advanced users to do their own thing.
The thing is that the official download is a 2-step process: retrieve the latest version, then the ISO based on a path containing the latest version. I wanted the new flag to be more of an 1-step override, bypassing the official release scheme, since it can take time to align to upstream (and one might want to pin to a specific version anyway).