OPENDJ-1778 Tools --help display the java class name instead of the command name

This is a problem since switching to using the opendj-cli argument classes.
This happens because the opendj-cli tools check the new "com.forgerock.opendj.ldap.tools.scriptName" property instead of the opends property "org.opends.server.scriptName" that is still set by the server tools.
Fixed by also checking the legacy opends property.
Factorized the code from ArgumentParser, SubCommandArgumentParser and DSConfig.


ArgumentParser.java:
Added constant PROPERTY_SCRIPT_NAME_LEGACY.
Extracted methods getScriptNameOrJava(), getLocalizableScriptName() and getScriptName().