Add trigger for terraform state for plan jobs

[cweibel]

Changes proposed in this pull request:

• Will help with detection of IP CIDR range changes for S3.
• Once the corresponding "plan" and "apply" jobs are run for an environment in cg-provision's pipeline, those will update the terraform statefile in S3. Changes to the statefiles are then used as data inputs for terraform in this repo. The proposed change here simply looks for updates to the statefile and fires the corresponding environment's "plan" step. It will still need the operator to review the changes and run the "apply" step so the application security groups are updated with the new ip ranges.
• This should close out https://github.com/cloud-gov/private/issues/1438

Security considerations

No real changes to security, simply quickens the response between when AWS changes IP CIDR ranges and when we reflect these changes within CF ASGs.