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/templates/virtual-mcp11-k8s-calico/salt.yaml b/tcp_tests/templates/virtual-mcp11-k8s-calico/salt.yaml
index b98a23a..a650b87 100644
--- a/tcp_tests/templates/virtual-mcp11-k8s-calico/salt.yaml
+++ b/tcp_tests/templates/virtual-mcp11-k8s-calico/salt.yaml
@@ -22,6 +22,8 @@
 
 {{ SHARED.ADJUST_K8S_OPTS() }}
 
+{{ SHARED.ADJUST_SL_OPTS(OVERRIDES_FILENAME='/srv/salt/reclass/classes/cluster/' + SHARED.CLUSTER_NAME + '/stacklight/server.yml') }}
+
 {%- if ENABLE_COMPUTES_SELF_REGISTER != '' %}
 {{ SHARED.REGISTER_COMPUTE_NODES() }}
 {%- endif %}