update ci configuration to send slack notifications on failure

Updates CI config file and adds a task to send notifications when a build fails on master or on any tag.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/12805

scripts/notify_slack.sh

+#!/bin/bash
+# Sends Slack notification ERROR_MSG to CHANNEL
+# An env. variable CI_SLACK_WEBHOOK_URL needs to be set.
+
+CHANNEL=$1
+ERROR_MSG=$2
+
+if [ -z "$CHANNEL" ] || [ -z "$ERROR_MSG" ] || [ -z "$CI_SLACK_WEBHOOK_URL" ]; then
+    echo "Missing argument(s) - Use: $0 channel message"
+    echo "and set CI_SLACK_WEBHOOK_URL environment variable."
+else
+    curl -X POST --data-urlencode 'payload={"channel": "'"$CHANNEL"'", "username": "gitlab-ci", "text": "'"$ERROR_MSG"'", "icon_emoji": ":gitlab:"}' "$CI_SLACK_WEBHOOK_URL"
+fi
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