Documentation: OpenStack image should have python installed

[rdefauw]

I'm using kargo to deploy on OpenStack. I used an Ubuntu 16.04 server image. When I first tried to run 'kargo deploy' the SSH check failed. It's actually the ansible_ping task which requires that python be installed on the VMs.

The docs should include a warning about this (or there should be a playbook to enforce the minimum requirements on the VMs).

[Smana]

ok to be added in the requirements part of the documentation