Docker-machine fetches obsolete boot2docker iso by default

[abelcheung]

Problem is noticed when docker-machine creates the initial VM under Windows host. It completely ignores bundled ISO image (but I have installed toolbox2docker in non-default location under D:\something, not sure if it matters), and tries to pull ISO from the now-archived boot2docker. It only allowed me to use the new xfs enabled ISO with --virtualbox-boot2docker-url option specified.

While patching and recompiling docker-machine is a very hefty price to pay for small problem like this. how about a few forewarning words in README?

[kaosagnt]

I knew about this but hadn't decided what to do. I haven't wanted to maintain a fork of yet another docker component before coming across the Gitlab fork of docker-machine with has more issues fixed but still will fetch the obsolete boot2docker ISO. I'll add the note to the README. Thanks for the report.