The previously hardcoded lxc path prevented the sudo wrapper from
working in environment with alternative `lxcpath`.
I had to move `sudo_wrapper` from `provider` to `LXC` because the
concept of "provider" is tied to a machine when a command sush as
`sudoers` is not.
Fixes#413 and #399
Sudoers now creates a safe wrapper script that performs sanity checks on sudo :
* wrapper generated in /usr/local/bin (name includes version to allow multiple wrappers on the same system)
* sudoers command now generates a one-line file in /etc/sudoers.d
* SudoWrapper use the new wrapper
* Removed unused Config#validate method
