Terraform-validate

- Fix all error messages to ensure that they provide the correct information. All error messages are now tested.

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- Fix the should_have_properties and should_not_have_properties to work properly when given a string thats not in a list

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- Resource matching done via a string will now work as a regex. Lists passed into `.resource` will work as before.
- Regex matching is more strict, will automatically enclose strings with ^ and $ characters.

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- Updated validation function names
- Fixed boolean:string comparison in `.should_equal()` and `.should_not_equal()`

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- Re-added `.disable_variable_expansion()` as this function was unintentionally deleted.

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- Rewrite of assertion code to split into different classes with objects that can chain together. This should result in tests that are easier to understand.
- Variables in strings are not expanded by default anymore. Use `.enable_variable_expansion()`` to expand variables.
- assert_* functions are deprecated, but can still be used.
- New error_if_property_missing() function will cause the validator to raise an exception if no matching properties were found in a resource.

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