This solution applies only to Ubuntu 16.04 LTS, where DNS leaks have appeared. Editing each and every .ovpn file with a script as advised by others, may work, it may not. For me it did not. If you have been doing this, replace the ovpn files with fresh ones and proceed.
If you have been mucking around for quite some time with this problem, a fresh install could be indicated, to which you can immediately implement this simple solution successfully.
This solution, may work for other Distro derivations to 16.04 LTS, that‘s possible, I have not tested this. This solution presumes you‘ve been successful in importing VPN config files and applied them only to discover a DNS leak of your IP's DNS server has shown up in a DNS Leak test.
What this solution does is force your computer to use only the DNS server as provided by your VPN provider's ovpn file.
In the terminal:
<enter your password>
apt-get install openresolv nscd unbound
Allow the install to proceed past the reboot recommendation. When finished, close the terminal.
For good measure, reboot your computer, start up your VPN and check with a DNS leak test. You should see only your VPN‘s DNS server listed. If you see your IP‘s DNS server, check your other VPN config files to see if this isn‘t just an aberation with your VPN provider‘s server.