Fix meta/salt.yml to workaround broken formulas

Change-Id: If8ee6ccfd282d8f926eff268cd74871e0f400754

salt/map.jinja

   virt_pkgs:
   - libvirt-dev
   - pkg-config
-{% if grains.oscodename == 'trusty' %}
+{% if grains.get('oscodename') == 'trusty' %}
   - libguestfs-tools
 {% endif %}
 

salt/meta/salt.yml

   {%- macro load_grains_file() %}{% include grains_fragment_file ignore missing %}{% endmacro %}
   {%- set grains_yaml = load_grains_file()|load_yaml %}
   {%- if grains_yaml is mapping %}
-    {%- for node in grains_yaml.graph %}
+    {%- for node in grains_yaml.graph if grains_yaml.graph %}
     {%- do service_grains.salt.graph.append(node) %}
     {%- endfor %}
   {%- endif %}