ignore_external_function_updates is not used

Found a bug? Maybe our [Slack Community](https://slack.cloudposse.com) can help.

[](https://slack.cloudposse.com)

## Describe the Bug
I was trying to use `ignore_external_function_updates` to set ignore on source changes and foudn that this variable is not used anywhere in the code.

## Expected Behavior
It actually changes behaviour :)

## Steps to Reproduce
* Set to `true` and see nothing changes
*
## Environment (please complete the following information):

Anything that will help us triage the bug will help. Here are some ideas:
- OS: all
- Version 0.4.1

## Additional Context
Add any other context about the problem here.

Search the Terraform module for the declaration and references to `ignore_external_function_updates`, then trace how source changes are handled when it is set to `true`. Reproduce the reported behavior from version 0.4.1 and confirm that the setting changes whether external function updates are ignored.

Written by the indexing model from the issue text.