fix(tofs): use source_files instead of files

[myii]

• fix(tofs): use source_files instead of files template-formula#76
• Upgrade formula to standards #2

---

Testing before and after, using a pillar containing:

  tofs:
    files_switch:
      - any/path/can/be/used/here
      - id
      - osfinger
      - os
      - os_family
    dirs:
      files: files_alt
      default: default_alt
    source_files:
      lighttpd_config:
        - '/etc/lighttpd/lighttpd_alt.conf'

Confirmed the same source output:

    - source:
      - salt://lighttpd/files_alt/any/path/can/be/used/here/etc/lighttpd/lighttpd_alt.conf
      - salt://lighttpd/files_alt/ABC/etc/lighttpd/lighttpd_alt.conf
      - salt://lighttpd/files_alt/Ubuntu-16.04/etc/lighttpd/lighttpd_alt.conf
      - salt://lighttpd/files_alt/Ubuntu/etc/lighttpd/lighttpd_alt.conf
      - salt://lighttpd/files_alt/Debian/etc/lighttpd/lighttpd_alt.conf
      - salt://lighttpd/files_alt/default_alt/etc/lighttpd/lighttpd_alt.conf

[aboe76]

@myii thanks for this