To resolve the warnings, we:
1. Use import_playbook instead of include for the conditional
playbook inclusions in site.yml. TIL that using include_playbook
does not work with conditionals.
2. Use include_tasks instead of include for the openstack-image-setup
task set inclusion.
3. Switch to using 'is' instead of '|' for tests.
Change-Id: I6d68bd4fecda122a77f7934842c3479a4c0792fd