Make possible to override the docker images for SL

Use the macro in the salt.yaml:
{{ SHARED.ADJUST_SL_OPTS(OVERRIDES_FILENAME='path/to/stacklight/server.yml') }}
and the following environment variables for image paths:

DOCKER_IMAGE_ALERTMANAGER
DOCKER_IMAGE_PUSHGATEWAY
DOCKER_IMAGE_PROMETHEUS
DOCKER_IMAGE_REMOTE_AGENT
DOCKER_IMAGE_REMOTE_STORAGE_ADAPTER
DOCKER_IMAGE_PROMETHEUS_RELAY
DOCKER_IMAGE_GRAFANA

, or manage the tag of the images using the single
variable:

DOCKER_IMAGES_SL_TAG= # latest, 2017.12, ...

Change-Id: Icf5039d9346f631ffb4391588b5090396268bc53
diff --git a/tcp_tests/settings_oslo.py b/tcp_tests/settings_oslo.py
index dacdb22..9ff0e40 100644
--- a/tcp_tests/settings_oslo.py
+++ b/tcp_tests/settings_oslo.py
@@ -197,20 +197,26 @@
            help="Path to YAML with steps to deploy sl",
            default=_default_sl_prepare_tests_steps_path),
     ct.Cfg('docker_image_alertmanager', ct.String(),
-           default='{}/openstack-docker/alertmanager:latest'.format(
-               settings.DOCKER_REGISTRY)),
+           default='{0}/openstack-docker/alertmanager:{1}'.format(
+               settings.DOCKER_REGISTRY, settings.DOCKER_IMAGES_SL_TAG)),
     ct.Cfg('docker_image_pushgateway', ct.String(),
-           default='{}/openstack-docker/pushgateway:latest'.format(
-               settings.DOCKER_REGISTRY)),
+           default='{0}/openstack-docker/pushgateway:{1}'.format(
+               settings.DOCKER_REGISTRY, settings.DOCKER_IMAGES_SL_TAG)),
     ct.Cfg('docker_image_prometheus', ct.String(),
-           default='{}/openstack-docker/prometheus:latest'.format(
-               settings.DOCKER_REGISTRY)),
+           default='{0}/openstack-docker/prometheus:{1}'.format(
+               settings.DOCKER_REGISTRY, settings.DOCKER_IMAGES_SL_TAG)),
     ct.Cfg('docker_image_remote_agent', ct.String(),
-           default='{}/openstack-docker/telegraf:latest'.format(
-               settings.DOCKER_REGISTRY)),
+           default='{0}/openstack-docker/telegraf:{1}'.format(
+               settings.DOCKER_REGISTRY, settings.DOCKER_IMAGES_SL_TAG)),
     ct.Cfg('docker_image_remote_storage_adapter', ct.String(),
-           default='{}/openstack-docker/remote_storage_adapter:latest'.format(
-               settings.DOCKER_REGISTRY)),
+           default='{0}/openstack-docker/remote_storage_adapter:{1}'.format(
+               settings.DOCKER_REGISTRY, settings.DOCKER_IMAGES_SL_TAG)),
+    ct.Cfg('docker_image_prometheus_relay', ct.String(),
+           default='{0}/openstack-docker/prometheus_relay:{1}'.format(
+               settings.DOCKER_REGISTRY, settings.DOCKER_IMAGES_SL_TAG)),
+    ct.Cfg('docker_image_grafana', ct.String(),
+           default='{0}/mirantis/external/grafana:{1}'.format(
+               settings.DOCKER_REGISTRY, settings.DOCKER_IMAGES_SL_TAG)),
     # SalesForce connection options for pushkin
     ct.Cfg('sfdc_sandbox_enabled', ct.String(), default='False'),
     ct.Cfg('sfdc_auth_url', ct.String(), default=''),