DOCKER-754 dockerinit needs to be able to mount NFS volumes

[joshwilsdon]

This adds support for a "docker:nfsvolumes" metadata key that includes a stringified array of volumes to mount in the form:

[
    {nfsvolume: "192.168.128.6:/exports/mah_bukket", mountpoint: "/mah_bukket", readonly: true},
    {nfsvolume: "192.168.128.15:/exports/mah_other_bukket", mountpoint: "/mah_other_bukket"}
]

and mounts the nfsvolume to the mountpoint in the zone.