erp5_core: call password_tool change password on context and not portal

As password tool use the context to redirect the user the it should be called on the current context.
Ex: If the user is changing his password in the context of a website the former code was redirecting to the root of ERP5. In the case the website had a dedicated url it was not noticeable.

product/ERP5/bootstrap/erp5_core/SkinTemplateItem/portal_skins/erp5_core/PasswordTool_changeUserPassword.xml

@@ -51,7 +51,7 @@
         <item>
             <key> <string>_body</string> </key>
             <value> <string>REQUEST = context.REQUEST\n
-return context.getPortalObject().portal_password.changeUserPassword(password=REQUEST[\'password\'],\n
+return context.portal_password.changeUserPassword(password=REQUEST[\'password\'],\n
                                                                     password_confirmation=REQUEST[\'password_confirm\'],\n
                                                                     password_key=REQUEST[\'password_key\'],\n
                                                                     user_login=REQUEST.get(\'user_login\', None),\n