Add fake usermod command

Ansible's `user` module *requires* that the `usermod` command be
present, even if it would not actually create or modify the specified
user.  It does not actually execute it unless it needs to, so we can
stub it out.  Making it a symlink to `false` ensures that it will never
do anything, and will fail if its actually run.

rootfs/overlay/usr/bin/usermod

@@ -0,0 +1 @@
+false