launch systemd-logind if available
only launch logind in saucy and trusty
moar logind
use find to recursively umount /sys for bootstrap
escape croutonversion call in enter-chroot
add basic logind test
don't rbind /sys if it's already bind-mounted

If enter-chroot is called twice for the same chroot, we don't want
slave mounts like the systemd cgroup to get blown away.
test: ensure systemd cgroup is set up in chroot
enter-chroot: mkdir /sys if needed
only run logind test if release is supported
Merge pull request #699 from smibarber/logind

Launch systemd-logind if available