Move docker_volume_too.go to subdir to fix build

I got this during install:

contrib/docker_volume_tool.go:14: main redeclared in this block
	previous declaration at contrib/crashTest.go:121

Moving the file to a volume-tool subdirectory fixes this
VolumeSet: Rename directory from "volumes" to "loopback"

"volumes" is already used for the docker volumes
Image: Add runtime argument to Mount

We will need this later to get the mount method settings and the
VolumeSet
VolumeSet: Add .dockerinit in base image

We need this in all image to be able to mount in docker
VolumeSet: Add HasVolume

We need to be able to check if a particular image ID already
has an image in the volume set when we create the images.
Runtime: Add volume set and mount method checking

We add a GetMountMethod() method to check whether AUFS or device-mapper
is used. And in the device-mapper case we store a single VolumeSet
for the runtime
Image: Iniital support for Mount on device-mapper