zuul: collect cinderlib logs from tempest node(s) only

The functional tests are executed from the tempest node(s), which means
that the collection must happen from the same place, not all nodes;
otherwise the playbook code fails on pure compute nodes.

Change-Id: Ibe1796e1380add34b6d8f19606bb55fda13c3c08
diff --git a/playbooks/post-cinderlib.yaml b/playbooks/post-cinderlib.yaml
index f61775c..6384bd1 100644
--- a/playbooks/post-cinderlib.yaml
+++ b/playbooks/post-cinderlib.yaml
@@ -1,4 +1,4 @@
-- hosts: all
+- hosts: tempest
   vars:
     tox_envlist: functional
     zuul_work_dir: "{{ ansible_user_dir }}/{{ zuul.projects['opendev.org/openstack/cinderlib'].src_dir }}"