Boot2docker: Linux support

[chantra]

While provisioning a b2d vm on Linux works fine, it does not mount the user's home dir. With a bit of patching on both boot2docker and docker-machine, I got something working which will allow running docker from a linux host and properly accessing /home.
While in most cases, one could just run docker natively on the Linux host, it is convenient to be ale to isolate that in a VM (like when you want to hack on netlink, the networking stack)

A combo of the following patch make a working solution:
https://github.com/chantra/boot2docker/commit/84da70960509dc704b4d216f2634114f2986e497 on the b2d side
and
https://github.com/chantra/machine/commit/8a01911bb8d1a6a12b1a77e5684916a63ca74cb8
on the docker-machine side.

The issue is that we can't mount the host /home in the VM so I trick that by creating /hosthome in the VM and, if the mount exist, I am linking the subdirs from the host's /home to the vm's /home

Unless there is a docker home dir on the host, it should work just fine.

Also that should fix #775

[atomi]

Any way #1086 can make the 1.10 release? /cc @tianon

[dschep]

In similar vein to @atomi's comment, will #1086 ever be merged? It's a pretty annoying bug. I have to use this everytime I create a new VM.

[atomi]

@dschep I ended up forking, merging this PR and building the iso myself. It works fine using --virtualbox-boot2docker-url with docker-machine https://github.com/atomi/boot2docker

[nmgeek]

It's cruel and unusual punishment to leave this issue unresolved. Open source developers generally expect code to work first/best on Linux.