public void setSystemUsername(String systemUsername) {
        this.systemUsername = systemUsername;
    }
Used when initializing the default {@link LdapContextFactory}.  This property is ignored if a custom
<tt>LdapContextFactory</tt> is specified.

@param systemUsername the username to use when logging into the LDAP server for authorization.
@see JndiLdapContextFactory#setSystemUsername(String)
core/src/main/java/org/apache/shiro/realm/ldap/AbstractLdapRealm.java