Can you suggest a good open source control panel?

I'd recommend Webmin for this - it's free, open source, and easy to use. There's actually a great walkthrough for installing and configuring Webmin on Ubuntu 16.04 in the Linode Docs:

How to Install Webmin on Ubuntu 16.04

The official Webmin Documentation also offers some great info.

If you need any help along the way, feel free to post additional questions here on the Linode Community site or check out the Webmin Forum

I've been using CentOS Web Panel for some time now and find it works really well for my Linode hosted ecommerce WordPress website. It works with CentOS 6 and 7 (7 is recommended) and although it is easy enough to install with clear instructions on their website, they also offer a very cheap 'one off' installation service along with a few different tiers of ongoing professional support packages.

Once installed the built in free dns manager can be used to configure DNS for domains and you can use the auto SSL feature to set up LetsEncrypt free SSL certificates for domains that already point to your IP. You can add user accounts and configure packages from the root account and manage every aspect of your server's configuration.

As well as all of the hosting space management tools you'd expect to find, the user accounts can also be linked with Softaculous to allow users to install a huge selection of popular web apps with a couple of clicks.

I've been personally using CentOS Web Panel for over 18 months now and I'd recommend it to anyone.