Add set-src-dest-host role

The role will find out the available hosts and set
compute.migration_source_host and compute.migration_dest_host
options in tempest.conf if tempest_set_src_dest_host is set to
True.

Change-Id: Ic01970b100a7bb30d9c78229204c7de67681108c
diff --git a/playbooks/devstack-tempest.yaml b/playbooks/devstack-tempest.yaml
index 269999c..6d70bc3 100644
--- a/playbooks/devstack-tempest.yaml
+++ b/playbooks/devstack-tempest.yaml
@@ -20,6 +20,11 @@
       include_role:
         name: acl-devstack-files
 
+    - name: Set source and destination host
+      include_role:
+        name: set-src-dest-host
+      when: tempest_set_src_dest_host is defined and tempest_set_src_dest_host | bool
+
     - name: Run tempest cleanup init-saved-state
       include_role:
         name: tempest-cleanup