Add support for file external_auth special directives

Signed-off-by: Johannes Löthberg <johannes@kyriasis.com>

salt/files/master.d/f_defaults.conf

 {%- for auth, users in cfg_master['external_auth']|dictsort %}
   {{ auth }}:
 {%- for user, commands in users.items() %}
+{%- if user.startswith('^') %}
+    {{ user }}: {% raw %}'{% endraw %}{{ commands }}{% raw %}'{% endraw %}
+{%- else %}
     {{ user }}:
 {%- for command in commands %}
 {%- if command is mapping %}
       - {% raw %}'{% endraw %}{{ command }}{% raw %}'{% endraw %}
 {%- endif %}
 {%- endfor -%}
+{%- endif %}
 {%- endfor -%}
 {%- endfor -%}
 {%- endif %}