Adds a new 'os_distribution' variable which can be used to select the OS
distribution, with sensible defaults for other variables. The default
value is 'centos'.

This patch changes the default value for the '*_bootstrap_user'
variables from using the $USER environment variable to using fixed
defaults equal to the os_distribution variable. This aligns with the
standard user configured in most cloud images, and images created via
DIB.

Note that we are continuing to use a CentOS based IPA image, since we
have been unable to get IPA to build for Ubuntu.

Depends-On: https://review.opendev.org/c/openstack/kayobe-config-dev/+/788234

Change-Id: I9e10239f58fe209867116fa2e10f1ce74220b966
Story: 2004960
Task: 42323