Work through remaining steps of TOFS implementation after PR #398

[myii]

Transferring from https://github.com/saltstack-formulas/salt-formula/pull/398#issuecomment-499752525:

• [ ] README: Update to explain how TOFS can be used instead of pillar for master/minion configuration.
• [ ] salt.cloud: (After due consideration) Use the same method to allow TOFS to be used for these configurations: cloud.maps.d, cloud.profiles.d & cloud.providers.d.
• [ ] Deprecation step 1: Display deprecation warning when configuration is being done by pillar.
• [ ] Deprecation step 2: Make TOFS the default, making pillar-based opt-in.
• [ ] Deprecation step 3: Remove pillar-based method, with breaking change commit.
• [ ] Alongside deprecation (at some point): Remove code to clean up old _defaults.conf file if they have it around -- all .conf files must be permitted, should a user choose to use that filename.

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This comment is useful for explaining how TOFS can be used instead of pillar: https://github.com/saltstack-formulas/salt-formula/pull/398#issuecomment-499354351.

[windowsrefund]

After @myii helped me to get TOFS working, I plan on using my new understanding of this approach to update the README.