Refuse to change the chronos password if already set unless explicitly asked.

This adds an optional --force argument which is needed if one attempts to change the password on an image where it is already set.

BUG=chrome-os-partner:1460
TEST=manually tested

Change-Id: I56a95fe4d699ce02c7a68e5be14cc7dce0609a54

Review URL: http://codereview.chromium.org/4480001
2 files changed