This is based on the code we used for Docker Editions, and
will format an external drive, to be used for example for
`/var` for Docker image persistence.
It does not `mount` the drive yet, as splitting format and mount
gives better modularity.
Example yaml fragment:
```
- name: format
image: "mobylinux/format:097d4f22b20f976b1f89d8f0b8a5d074d35b856c"
binds:
- /dev:/dev
capabilities:
- CAP_SYS_ADMIN
- CAP_MKNOD
```
Signed-off-by: Justin Cormack <justin.cormack@docker.com>
