driver: exoscale driver

Add support for exoscale, a Swiss cloud provider. This pull "egoscale",
a Go binding for exoscale, in godeps.

Signed-off-by: Vincent Bernat <Vincent.Bernat@exoscale.ch>
driver/exoscale: fix homedir for Windows

Signed-off-by: Vincent Bernat <Vincent.Bernat@exoscale.ch>
driver/exoscale: fetch docker from get.docker.com repository

Instead of using Docker provided by the distribution, we use
get.docker.com repository to get the latest Docker.

This allows us to implement the "upgrade" method.

The cloud-init block "runcmd" will have to be removed when identity
authentication will be merged in Docker.

Signed-off-by: Vincent Bernat <Vincent.Bernat@exoscale.ch>