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/cookied-mcp-ocata-dvr-vxlan/underlay.yaml b/tcp_tests/templates/cookied-mcp-ocata-dvr-vxlan/underlay.yaml
index b63a123..ef002fe 100644
--- a/tcp_tests/templates/cookied-mcp-ocata-dvr-vxlan/underlay.yaml
+++ b/tcp_tests/templates/cookied-mcp-ocata-dvr-vxlan/underlay.yaml
@@ -344,7 +344,7 @@
           - name: {{ HOSTNAME_CFG01 }}
             role: salt_master
             params:
-              vcpu: !os_env SLAVE_NODE_CPU, 2
+              vcpu: !os_env SLAVE_NODE_CPU, 3
               memory: !os_env SLAVE_NODE_MEMORY, 4096
               boot:
                 - hd