Ensure boostrap_host_exec runs as root

Ensure boostrap_host_exec runs as root

This is necessary for accessing the bind mounted hieradata in the
container in order to determine if the node is the primary node.

With the new validation added to yaml-validate.py, we could spot
potential issues in sahara-api and keystone bootstrap tasks.

The keystone one is a false positive, as the image defaults to the root
user in order to be able to run apache. Still, it is better to be
consistent here and specify the root user nonetheless.

Change-Id: Ib0ff9748d5406f507261e506c19b96750b10e846
Closes-Bug: #1697917