Set backing store to use for lxc container

[tailhook]

Since it's a parameter to lxc-create not to template it's not possible to set the value neither in template options not in lxc config. So I'd expect default to be lxc-create --bdev best with an option to set some specific store in Vagrantfile

[fgrehm]

This is a dupe of GH-232 (which will hopefully be handled by GH-235). I just need to find the time to fix some other stuff before getting to those issues.