Don't mount /sys/fs/fuse/connections and /sys/kernel/config

These fail to mount if you don't have the appropriate kernel support,
and this confuses NixOps' ‘check’ command. We should teach NixOps not
to complain about non-essential mount points, but in the meantime it's
better to turn them off.

(cherry picked from commit 6eaced35820f4c575dd545fb721abd74f248c8cb)