Various changes for tomorrow's workshop

Things I modified:

 - Renamed 'command_host' and 'host' into 'master' and 'hypervisor' (this was confusing as host was sometimes used for target)
 - Use yes/no instead of true/false as a best practice
 - Reduce virtual disk image to 3GB (which should be plenty)
 - Renamed 'tree' and 'centos' repositories into 'minimal' and 'workshop'
 - Add 'base' and 'updates' repositories but leave them disabled
 - Don't remove all existing CentOS repositories (just leave the disabled ones)
 - Use a 32bit minimal CentOS image
 - Add various additional packages we need for the examples (e.g. git, httpd, vim-enhanced)
 - Use local_action for wait_for, instead of delegate_to
 - Changed root password to 'root' (works fine on qwerty and azerty)