Resolve issue with mount_points.readOnly bool and TF15

[darpham]

what

• Add backwards compatibility with TF15 related to mount_points variable

why

• Incompatibility with TF15, root cause is unknown

references

• closes Terraform 0.15 error when specifying bool input #136

comments/notes

• This will force users to specify the readOnly bool, previously where null was specified should now be false per AWS default
• Initially wanted to use default optional function within the variable, but this feature is only available in TF15

[nitrocode]

See the original PR https://github.com/cloudposse/terraform-aws-ecs-container-definition/pull/78/files that changed

Unfortunately this will break backwards compatibility with previous default of readOnly to false which is why the list(object()) definition for this variable was removed since in pre-0.15 versions, we could not set optional map values.

It would be better to analyze why terraform 15 has an issue with unmarshalling string readOnly of type bool. Perhaps it's a bug in terraform?

Even better would be that once we move to all of our modules to tf 15, we use the optional function instead and remove our dependence on previous versions.

[nitrocode]

/test all

[nitrocode]

@darpham it seems it's better to explicitly set the variable if there is no other workaround. Thank you for the PR.