fix(tofs): use source_files instead of files

[myii]

• fix(tofs): use source_files instead of files saltstack-formulas/template-formula#76
• fix(tofs): use tpldir derivative topdir for pillar (config) paths #1

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Also involved updating topdir to tplroot.

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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:
      stack-config:
        - 'sdb.conf'
        - 'sdb_alt.conf'
      stack-keys-config:
        - 'sdb_keys.conf'
        - 'sdb_keys_alt.conf'

Confirmed the same source output:

    - source:
      - salt://stack/files_alt/any/path/can/be/used/here/sdb.conf
      - salt://stack/files_alt/any/path/can/be/used/here/sdb_alt.conf
      - salt://stack/files_alt/ABC/sdb.conf
      - salt://stack/files_alt/ABC/sdb_alt.conf
      - salt://stack/files_alt/Ubuntu-16.04/sdb.conf
      - salt://stack/files_alt/Ubuntu-16.04/sdb_alt.conf
      - salt://stack/files_alt/Ubuntu/sdb.conf
      - salt://stack/files_alt/Ubuntu/sdb_alt.conf
      - salt://stack/files_alt/Debian/sdb.conf
      - salt://stack/files_alt/Debian/sdb_alt.conf
      - salt://stack/files_alt/default_alt/sdb.conf
      - salt://stack/files_alt/default_alt/sdb_alt.conf

    - source:
      - salt://stack/files_alt/any/path/can/be/used/here/sdb_keys.conf
      - salt://stack/files_alt/any/path/can/be/used/here/sdb_keys_alt.conf
      - salt://stack/files_alt/ABC/sdb_keys.conf
      - salt://stack/files_alt/ABC/sdb_keys_alt.conf
      - salt://stack/files_alt/Ubuntu-16.04/sdb_keys.conf
      - salt://stack/files_alt/Ubuntu-16.04/sdb_keys_alt.conf
      - salt://stack/files_alt/Ubuntu/sdb_keys.conf
      - salt://stack/files_alt/Ubuntu/sdb_keys_alt.conf
      - salt://stack/files_alt/Debian/sdb_keys.conf
      - salt://stack/files_alt/Debian/sdb_keys_alt.conf
      - salt://stack/files_alt/default_alt/sdb_keys.conf
      - salt://stack/files_alt/default_alt/sdb_keys_alt.conf