gistfile1.txt

[ david@samwise ] $ cat salt.sls 
/etc/salt/minion:
  file:
    - managed
    - owner: root
    - group: root
    - mode: 444
    - source: salt://salt/files/salt-minion.config
    - template: jinja
    - context: {
        modules_dirs:
        {% if grains['fqdn'] == 'clients.foo.bar'%}
        "/home/core/modules"
        {% else %}
        "/opt/salt/modules"
        {% endif %}
       }

[ david@samwise ] $ cat salt-minion.config 
master: ctl.foo.bar
modules_dirs: {{ modules_dirs }}