Reject ssh changes with invalid root password

Closes #155 

To test:
1. Open Administration > Advanced > System > Administration in a web browser
2. SSH into the node and run `uci show dropbear`
3. Using an incorrect/blank root password, change a number of settings on the page, including ssh keys.
4. Click Save, then run `uci show dropbear` from the command line. The settings should not have changed. If you added an ssh key, it should not have been added to /etc/dropbear/authorized_keys.
5. Repeat using an incorrect root password and Save & Apply instead of Save. Your settings should not appear in /etc/config/dropbear.
6. Enter the correct root password and repeat. Your new settings should have been saved.