OPENDJ-1934 UI: Create/Update VLV indexes

Add support for pluggable backend VLV indexes in control panel

* org.forgerock.opendj.adapter.server3x.Converters.java
** Add two methods to convert pluaggable/JE backend VLV index search scope from old to new config framework classes

* org.opends.guitools.controlpanel.datamodel.VLVIndexTypeDescriptor.java
** Rename method getPluggableBackendVLVIndexScope(...) method to getBackendVLVIndexScope(...) in order to be more consistent with config framework classes

* org.opends.guitools.controlpanel.ui.AbstractVLVIndexPanel.java
** Add methods to create/update VLV indexes offline using the new configuration framework
** Remove hardcoded LDIF
** Use the SDK DN class instead of the legacy one

* org.opends.guitools.controlpanel.ui.VLVIndexPanel.java
** Add the pluggable backend indexes support in existing modifyIndex(...) method
** Rename modifyIndex() to modifyVLVIndexOffline()
** Rename modifyIndex(InitialLdapContext ctx) to modifyVLVIndexOnline(InitialContext ctx)
** Change the method process to avoid hardcoded LDIF code (getIndexLDIF() and inner class attribute) usage.
** Use inner class attribute when it is possible
** Remove useless comment

* org.opends.guitools.controlpanel.ui.NewVLVIndexPanel.java
** Add the pluggable backend VLV indexes support in existing createIndex(...) method
** Rename createIndex() to createVLVIndexOffline()
** Rename createIndex(InitialLdapContext ctx) to createVLVIndexOnline(InitialLdapContext ctx)
** Change the method process to avoid hardcoded LDIF code (getIndexLDIF() and inner class attribute) usage.
** Use inner class attribute when it is possible