create a horizon exercise for sanity check

from time to time horizon gets broken in devstack and it
takes a while to figure out why. Put a sanity check into
devstack exercises that checks for horizon front page being
up and not just a stack trace to use as a simple gate.

Change-Id: I13a6c59881f618d5194b1625b67115013c9cb6c2

exercises/horizon.sh

@@ -0,0 +1,45 @@
+#!/usr/bin/env bash
+
+# **horizon.sh**
+
+# Sanity check that horizon started if enabled
+
+echo "*********************************************************************"
+echo "Begin DevStack Exercise: $0"
+echo "*********************************************************************"
+
+# This script exits on an error so that errors don't compound and you see
+# only the first error that occured.
+set -o errexit
+
+# Print the commands being run so that we can see the command that triggers
+# an error.  It is also useful for following allowing as the install occurs.
+set -o xtrace
+
+
+# Settings
+# ========
+
+# Keep track of the current directory
+EXERCISE_DIR=$(cd $(dirname "$0") && pwd)
+TOP_DIR=$(cd $EXERCISE_DIR/..; pwd)
+
+# Import common functions
+source $TOP_DIR/functions
+
+# Import configuration
+source $TOP_DIR/openrc
+
+# Import exercise configuration
+source $TOP_DIR/exerciserc
+
+is_service_enabled horizon || exit 55
+
+# can we get the front page
+curl http://$SERVICE_HOST 2>/dev/null | grep -q '<h3>Log In</h3>' || die "Horizon front page not functioning!"
+
+set +o xtrace
+echo "*********************************************************************"
+echo "SUCCESS: End DevStack Exercise: $0"
+echo "*********************************************************************"
+