feat(boot_nspawn): Add wrapper script for booting via systemd-nspawn

This script will boot the build target root via systemd-nspawn which is
a mighty bit faster than building and booting vm images. :)

Note: systemd-nspwan doesn't do anything special for networking so port
conflicts for things like sshd are to be expected. Works though. :)
Merge pull request #2 from marineam/nspawn

feat(boot_nspawn): Add wrapper script for booting via systemd-nspawn