Complete Nextcloud installation with plugin

The Nextcloud plugin requires the user to complete the Nextcloud installation process at the Nextcloud web interface before it will work. This means the user needs to note usernames/passwords/paths from the pop-up in the FreeNAS web GUI, which is tedious, undocumented, and unnecessary.

https://github.com/freenas/iocage-plugin-nextcloud/pull/13 resolves this by using the occ command to complete the installation, giving the user a ready-to-use Nextcloud installation.