Issue #2025: Mention new var to override for older linux versions.

Author: geerlingguy

docs/configurations/base-os.md

@@ -13,21 +13,25 @@ For certain OSes, there are a couple other caveats and tweaks you may need to pe
 
 Some other OSes should work, but are not regularly tested with Drupal VM, including Debian 8/Jessie (`debian/jessie64`).
 
-## Ubuntu 18.04 Xenial LTS
+## Ubuntu 18.04 Bionic LTS
 
-Everything should work out of the box with Ubuntu 16.04.
+Everything should work out of the box with Ubuntu 18.04.
 
 ## Ubuntu 16.04 Xenial LTS
 
-Everything should work out of the box with Ubuntu 16.04.
+Most everything should work out of the box with Ubuntu 16.04. You will need to override one variable in your `config.yml` to use an older version of Python when provisioning:
+
+    ansible_python_interpreter: /usr/bin/python
 
 ## RedHat Enterprise Linux / CentOS 8
 
 Everything should work out of the box with RHEL 8.
 
 ## RedHat Enterprise Linux / CentOS 7
 
-Everything should work out of the box with RHEL 7.
+Most everything should work out of the box with CentOS 7. You will need to override one variable in your `config.yml` to use an older version of Python when provisioning:
+
+    ansible_python_interpreter: /usr/bin/python
 
 ## Debian 10 Buster