fix(update-ssh-keys): Tweak help text.

There is a -D, not just -a and -d.
fix(update-ssh-keys): Tweak add behavior, add -A to counter -D.

Now -a will never replace a zero-length (disabled) file regardless of
whether -n was passed or not. -A will enable but still respects -n.
fix(addon_config): Update existing oem ssh keys unless disabled.

Rely on the new update-ssh-keys behavior to handle enable/disable.
Merge pull request #15 from marineam/enable

Better enable/disable behavior for update-ssh-keys