OPENDJ-1932 OPENDJ-1933 (CR-6839) New Backend in UI

This change allows a user to create a new base DN and/on a new pluggable backend from the control panel.

* org.opends.guitools.controlpanel.ui.NewBaseDNPanel
** Improves UI design a bit
** Adds a combo box and a label to allow a user to choose a backend type
** Extracts method to manage the creation of a new backend and/or the creation of a new base DN.

* org.opends.server.tools.BackendCreationHelper
** Creates this new class to factorize backend creation code
** Moves DefaultIndex inner class from ConfigureDS
** Moves DEFAULT_INDEXES from ConfigureDS

* org.opends.server.tools.BackendTypeHelper
** Creates a new (ugly) method to retrieve a old configuration framework backend object corresponding from the same new configuration framework object. This allows to factorize backend creation code.
** Moves the new configuration framework initialization to Utilities.

* org.opends.server.tools.ConfigureDS
** Consequences of creation of BackendCreationHelper.

* org.opends.guitools.controlpanel.util.Utilities
** Migrates the new config framework initialization as it is use in many places now.

* org.opends.messages.admin_tools.properties
** Add two new messages