Terraform-validate

- Attempt to calculate a variable value if it is enclosed in `upper()` or `lower()` interpolation functions

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- Fixed bug that caused asserts to throw errors when there were no "resource" configurations present in a terraform project

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- Addition of new Asserts that validate Variable default values
- assert_variable_default_value_exists
- assert_variable_default_value_equals
- assert_variable_default_value_matches_regex

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- Initial Release

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