fixes #108: persist identity keys in b2d

This changes where the identity keys are stored in b2d.  This persists through reboots and stop/starts.

Note: I also created a new b2d iso for testing as the current one had issues with TLS and ports.

ISO: https://ehazlett.s3.amazonaws.com/public/boot2docker/boot2docker-machine-87e3813.iso

```
machine create -d virtualbox
    --virtualbox-boot2docker-url https://ehazlett.s3.amazonaws.com/public/boot2docker/boot2docker-machine-87e3813.iso 
    machine-test
```
I agree. I've refactored it to be slightly less suck.

On Tuesday, December 16, 2014, Nathan LeClaire <notifications@github.com>
wrote:

> In drivers/virtualbox/virtualbox.go
> <https://github.com/docker/machine/pull/111#discussion-diff-21942330>:
>
> > +		return err
> > +	}
> > +
> > +	if err := drivers.AddPublicKeyToAuthorizedHosts(d, "/var/lib/boot2docker/.docker/authorized-keys.d"); err != nil {
> > +		return err
> > +	}
> > +
> > +	// HACK: configure docker to use persisted auth
> > +	cmd, err = d.GetSSHCommand("echo DOCKER_TLS=no | sudo tee -a /var/lib/boot2docker/profile")
> > +	if err != nil {
> > +		return err
> > +	}
> > +	if err := cmd.Run(); err != nil {
> > +		return err
> > +	}
> > +	cmd, err = d.GetSSHCommand("echo \"EXTRA_ARGS='--auth=identity --auth-authorized-dir=/var/lib/boot2docker/.docker/authorized-keys.d --auth-known-hosts=/var/lib/boot2docker/.docker/known-hosts.json --identity=/var/lib/boot2docker/.docker/key.json -H tcp://0.0.0.0:2376'\" | sudo tee -a /var/lib/boot2docker/profile")
>
> Cool. Eventually I would think that some of this stuff will be baked into
> the b2d iso too, but as long as we're supporting boot2docker-cli *and*
> machine it seems acceptable to do this sort of logic here.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/docker/machine/pull/111/files#r21942330>.
>