xenapi: Use a jeos vm as a template

DevStack was using templating to speed up the setup process with
XenServer. The template already included some devstack customisations,
not just a clean OS. This change modifies devstack behaviour, so that
the template is a simple clean operating system. This makes it easier to
use custom OS as a template, potentially speeding up the tests.

related to blueprint xenapi-devstack-cleanup

Change-Id: I6cb0a7ed7a90e749b78329a8e2b65fb8b7fcfa5f
Merge "xenapi: Use a jeos vm as a template"