conf/debian-jessie: Use consistent /proc, /sys and /sys/fs/cgroup (v2)

Without this a systemd container won't start:
> $ lxc-start -n vagrant-base-stretch-amd64
> Failed to mount cgroup at /sys/fs/cgroup/systemd: Permission denied
> [!!!!!!] Failed to mount API filesystems, freezing.
> Freezing execution.

See upstream commit: lxc-1.1.0.rc3-12-gf24a52d [Use consistent /proc, /sys and /sys/fs/cgroup (v2)]

conf/debian-jessie

@@ -6,8 +6,7 @@ lxc.kmsg = 0
 lxc.pivotdir = lxc_putold
 
 # Default mount entries
-lxc.mount.entry = proc proc proc nodev,noexec,nosuid 0 0
-lxc.mount.entry = sysfs sys sysfs defaults 0 0
+lxc.mount.auto = cgroup:mixed proc:mixed sys:mixed
 
 # Default console settings
 lxc.tty = 4