[kolla] Following Nova with XenAPI deprecation

Hello fellow OpenStackers! There exists some logic in Kolla Ansible that handles XenAPI-specific overrides and bootstrapping with os-xenapi. It is one of those murky, untested features that we prefer to remove as soon as possible. :-) Hence we follow Nova in deprecating this functionality for removal. We will do the removal in Victoria. If you are using XenAPI with recent Kolla-Ansible, please let us know. I will be proposing deprec changes later today. As far as other deployment projects are concerned, DevStack, OpenStack-Helm and Puppet-OpenStack seem to still handle XenAPI specifics. -yoctozepto