Exclude require statement when install_packages is False.

The 'require' statement needs a list as input, but doesn't get
any input when install_packages is set to False.

salt/ssh.sls

     - name: {{ salt_settings.config_path }}/roster
     - source: salt://salt/files/roster.jinja
     - template: jinja
-    - require:
 {% if salt_settings.install_packages %}
+    - require:
       - pkg: ensure salt-ssh is installed
 {% endif %}