provisiond: failed to mount volume in container when using coreX

[zaibon]

Problem seems to come from the fact that coreX does a chroot after it stars which somehow mask the bind mount in the container.

This chroot approach was taken cause we needed to have all the libraries and binaries of coreX in the container, so when using coreX, the container is started with /var/lib/corex as root filesystem, the flist mounted at /sandbox and the a chroot was done into /sandbox to have the root of the flist as root of the container.

This all hack seems to have reached his limit.
The proposed solution for this is to have a static build if coreX that we could easily insert into the container when needed. That would remove the need of starting the container with /user/lib/corex as root and avoid the chroot too.

CoreX status build issue: threefoldtech/corex#13