overlay should be higher priority than devicemapper

[SamSaffron]

we know for a fact devicemapper still has issues, we also know it is far more likely overlay is far more likely to work

[GordonTheTurtle]

Can you please sign your commits following these rules:
https://github.com/docker/docker/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "patch-1" git@github.com:SamSaffron/docker.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

This will update the existing PR, so DO NOT open a new one.

[jessfraz]

this is already in progress there are some really big bugs with overlay that need to be fixed in the kernel first #12354