JCRVLT-26 File vault stores passwords in clear text in ~/.vault/auth.xml

- adding new option --update-credentials
- adding prompt for password if missing in --credentials
- avoid storing credentials if not explicitely indicated (except admin:admin)

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/commons/filevault/trunk@1560840 13f79535-47bb-0310-9956-ffa450edef68