make start user dn configurable
some use cn= or uid= or sn= etc..
so it's now configurable with default uid=

git-svn-id: https://svn.apache.org/repos/asf/archiva/redback/redback-core/trunk@1436669 13f79535-47bb-0310-9956-ffa450edef68
prevent NPE

git-svn-id: https://svn.apache.org/repos/asf/archiva/redback/redback-core/trunk@1436670 13f79535-47bb-0310-9956-ffa450edef68
LdapUserManagr can be not readOnly depending on configuration

git-svn-id: https://svn.apache.org/repos/asf/archiva/redback/redback-core/trunk@1436671 13f79535-47bb-0310-9956-ffa450edef68