xci: playbooks: Add task for loading distribution variables

xci: playbooks: Add task for loading distribution variables

vars_files doesn't support loading files with variables in their names.
As such we add a new task to include these files before running any of
the roles. This fixes warnings like this:

Info: Starting provisining VM nodes using openstack/bifrost
-------------------------------------------------------------------------
No config file found; using defaults
skipping vars_file '../var/{{ ansible_os_family }}.yml' due to an undefined variable
skipping vars_file '../var/{{ ansible_os_family }}.yml' due to an undefined variable
skipping vars_file '../var/{{ ansible_os_family }}.yml' due to an undefined variable
skipping vars_file '../var/{{ ansible_os_family }}.yml' due to an undefined variable
skipping vars_file '../var/{{ ansible_os_family }}.yml' due to an undefined variable
skipping vars_file '../var/{{ ansible_os_family }}.yml' due to an undefined variable

Change-Id: I0a49e2ed8d811bb40a99612462752f8691133e0f
Signed-off-by: Markos Chandras <mchandras@suse.de>