OPENDJ-1714 (CR-6533) Add backend type choice in setup cli

* opendj-cli/src/main/java/com/forgerock/opendj/cli/ArgumentConstants.java
** Add backend type short and long optiion

* opendj-core/src/main/java/org/forgerock/opendj/ldap/MemoryBackend.java
** Change code to allow root DSE sub-entriees

* opendj-server-legacy/resource/config/config.ldif
** Remove harcoded userRoot backend ldif definition

* opendj-server-legacy/src/main/java/org/opends/quicksetup/UserData.java
** Add backend type argument

*  opendj-server-legacy/src/main/java/org/opends/quicksetup/installer/Installer.java
** Forward backend type argument to ConfigureDS

* opendj-server-legacy/src/main/java/org/opends/server/backends/pluggable/BackendImpl.java
** Check if storage db directory exists before trying to remove all storage files

* opendj-server-legacy/src/main/java/org/opends/server/tools/ConfigureDS.java
** Adapt the way to configure server base DNs

* opendj-server-legacy/src/main/java/org/opends/server/tools/InstallDS.java
** Add method to retrieve all available backend types from the config framework
** Adapt console UI to add the backend type choice

* opendj-server-legacy/src/main/java/org/opends/server/tools/InstallDSArgumentParser.java
** Add the backend type argument

* opendj-server-legacy/src/messages/org/opends/messages/tool.properties
** Add messages related to console UI