Piper/config simplification

[pipermerriam]

This is part of #395

What was wrong?

• Populus level config was unnecessarily written to disk when not present. Non-ideal default behavior as it makes upgrading harder when nothing has changed, requiring that we overwrite the previous config.
• Project level config doesn't actually need to exist since it's just defaults. Users can fill this in with config values when they need them.
• References within config files are problematic.

How was it fixed?

This is a first step towards fixing these issues.

• Remove all config values from project config file that are default, or remove the whole file if it's nothing but defaults.
• Remove all config values from populus config file that are default, or remove the whole file if it's nothing but defaults.
• Remove the functionality that creates the populus config file.
• Remove support for references and deference all existing references.

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