Make image path for tempest configurable

Testing image must be configurable in tempest config scripts.
Set it default for temporary github mirror until
1713358 is resolved.
Related-Bug: #1713358

Change-Id: Idc979d60e7ed5d36646d2a9c3fd41e5f85ee15ca

roles/validate-tempest/defaults/main.yml

@@ -38,3 +38,4 @@ tempest_version_dict: { 'newton': 'newton', 'ocata': '16.0.0', 'pike': '16.1.0',
 tempest_version: "{{ tempest_version_dict[release] }}"
 tempestmail_config: config.yaml
 tempest_track_resources: true
+tempest_test_image_path: http://download.cirros-cloud.net/0.3.5/cirros-0.3.5-x86_64-disk.img

roles/validate-tempest/templates/configure-tempest.sh.j2

@@ -139,6 +139,7 @@ cd {{ working_dir }}/tempest
 ${TEMPESTCONF} --out etc/tempest.conf \
   --network-id $public_net_id \
   --deployer-input ~/{{ tempest_deployer_input_file }} \
+  --image {{ tempest_test_image_path }} \
   --debug --create \
   identity.uri $OS_AUTH_URL \
   {% if release in ['newton','ocata'] %}