fix(tofs): prepend the config-based source_files to the default

* saltstack-formulas/nginx-formula#247 (comment)
  - The main issue is that the `nginx-formula` has dynamic values being
    used by as the default `source_files` -- there is no way to provide
    this from the pillar/config in a sensible fashion
  - Prepending to this default (rather than overriding it) resolves this
    problem entirely, without adding excessive entries to the `source`
* Closes saltstack-formulas#151

template/libtofs.jinja

@@ -10,7 +10,7 @@
 
     Params:
       * source_files: ordered list of files to look for
-      * lookup: key under '<tplroot>:tofs:source_files' to override
+      * lookup: key under '<tplroot>:tofs:source_files' to prepend to the
         list of source files
       * default_files_switch: if there's no config (e.g. pillar)
         '<tplroot>:tofs:files_switch' this is the ordered list of grains to
@@ -55,14 +55,13 @@
       tplroot ~ ':tofs:files_switch',
       default_files_switch
   ) %}
-  {#- Lookup source_files (v2), files (v1), or fallback to source_files parameter #}
+  {#- Lookup source_files (v2), files (v1), or fallback to an empty list #}
   {%- set src_files = salt['config.get'](
       tplroot ~ ':tofs:source_files:' ~ lookup,
-      salt['config.get'](
-          tplroot ~ ':tofs:files:' ~ lookup,
-          source_files
-      )
+      salt['config.get'](tplroot ~ ':tofs:files:' ~ lookup, [])
   ) %}
+  {#- Append the default source_files #}
+  {%- set src_files = src_files + source_files %}
   {#- Only add to [''] when supporting older TOFS implementations #}
   {%- set path_prefix_exts = [''] %}
   {%- if v1_path_prefix != '' %}