No www-data:www-data is user:group that Apache/SuExec uses to access file system - if I explained it correctly.
Bottom line is:
If you want to install modules/components/extensions in Joomla you should have all your folders and files chown-ed to www-data:www-data.
Except images folder where you will upload pictures - this folder can be web1_ftp-web1:web1_ftp-web1
If installation from zip files doesn’t work you can unpack desired extension in some folder and go with install from directory procedure.
This depends what Joomla are you using and what Linux distribution have you installed.