Add missing dependency to Horizon integration tests

The Horizon integration tests are based on Selenium and require Firefox
to run. Since the browser is not installed by default on the thin nodes
running devstack, add a new macro to do so.

Change-Id: Ie44c36663f8ad9909cf182f852767029b4523a76

jenkins/jobs/horizon.yaml

@@ -55,6 +55,7 @@
       - link-logs
       - net-info
       - devstack-checkout
+      - firefox-install
       - xvfb-install
       - shell: |
           #!/bin/bash -xe

jenkins/jobs/macros.yaml

@@ -88,6 +88,13 @@
           sudo apt-get update
           sudo apt-get install -y xvfb
 
+- builder:
+    name: firefox-install
+    builders:
+      - shell: |
+          sudo apt-get update
+          sudo apt-get install -y firefox
+
 - builder:
     name: uninstall-puppet
     builders: