OAK-50 : Implement User Management  (TODO in UserInitializer)

- default pw for the admin user is only used if the configuration doesn't provide a value
- if the configuration contains 'null' value the password property for the admin user is not set forcing
   applications to set the password before the admin can login.
- if the configuration param is missing the same behavior applies as in jr 2.x

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1471019 13f79535-47bb-0310-9956-ffa450edef68
OAK-50 : Implement User Management  (javadoc)

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1471020 13f79535-47bb-0310-9956-ffa450edef68