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
```
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.