feat(update-ssh-keys): Add -n option to skip updating existing keys.
fix(update-ssh-keys): Support add and del in the same command.

Just simplifies logic a little, don't expect to actually need this.
feat(update-ssh-keys): Add disable option to prevent adding with -n -a

Calling -D name will empty a key file instead of deleting it so a later
add command with -n (no replace) will skip adding the key.
fix(addon_config): Add no-replace option to ssh-update-keys

Now the user can disable the oem or other automatically installed key
and expect it to remain gone after the next boot.
Merge pull request #13 from marineam/disable

Disable automatically installed oem keys