lxc-template: make runnable by unprivileged users
lxc-template: make runnable by unprivileged users

lxc-template needlessly require root privileges in two places:

1. lock file location for `flock`
2. failing on `tar` failure during rootfs extraction

For `flock`, it's not necessary that the lock file be in `/var/lock`, it
can be anywhere. Why not put it in `LXC_PATH`?

For the failing `tar` thing, that's because some device are created with
`mknod` which unprivileged users can't do. These device, however, are
not necessary for the container to run well. We can ignore `tar`'s error
exit code.

I replaced the exist code check by a check for the existence of
`/bin/true` in rootfs. I think that it's a good indication of whether
the rootfs was extracted.

Why am I making this change? Because I'd like to add support for
unprivileged containers in `vagrant-lxc` but it's kind of a big change
to make at once, so I thought I'd go incrementally.

Val/vagrant-lxc,Val/vagrant-lxc,fgrehm/vagrant-lxc,fgrehm/vagrant-lxc,Val/vagrant-lxc,fgrehm/vagrant-lxc