tflint: Allow config to be merged even with initial values

[wata727]

See also #1604

The tflint.Config does not have a mechanism to distinguish between undefined and initial values, and whether or not it is overwritten was determined by whether it was different from the initial value.

tflint/tflint/config.go

Lines 331 to 333 in 16e0218

	if other.Module {
	c.Module = true
	}

However, this cannot override it to the initial value, and fails to achieve requirements like #1604.

This PR adds a flag to distinguish between undefined and initial values in tflint.Config. The tfllint.Config will only be overridden if the flag is set. With this change, the following five values can be overridden to the initial values:

• module
• force
• disabled_by_default
• plugin_dir
• format

However, since the initial values for these values cannot be set from the CLI, the current behavior does not change.