Delete Terraform plugins before pushing bbl state

[rowanjacobs]

What is this change about?

This change deletes Terraform plugins from the bbl state directory in the bbl-up task before pushing the state to GitHub or tarring and uploading it to S3/GCS/etc.

These plugin files are very large (the GCP one is around 50 MB), and bbl will re-download them as needed using terraform init before every operation, so it's not necessary to keep them on disk.

There was some discussion between us over whether this new behavior should be default or not. We introduced a new parameter, DELETE_TERRAFORM_PLUGINS, to the bbl-up task to control this new behavior. This is currently defaulting to true but we are open to changing it to default false as well.

Please provide contextual information.

This PR came out of an in-person discussion with @vitreuz and @acosta11.

Please check all that apply for this PR:

• [ ] introduces a new task
• [x] changes an existing task
• [ ] changes the Dockerfile
• [ ] introduces a breaking change (other users will need to make manual changes when this is released)

(my pair thinks this is a breaking change but I don't)

Did you update the README as appropriate for this change?

• [ ] YES
• [x] N/A

How should this change be described in release notes?

Delete Terraform plugins from the bbl-state repo before pushing it to Git or uploading it. This new behavior can be disabled.

What is the level of urgency for publishing this change?

• [x] Urgent - unblocks current or future work
• [ ] Slightly Less than Urgent

Tag your pair, your PM, and/or team!

cc @bruce-ricard cc @cloudfoundry/pcf-toolsmiths

[cfdreddbot]

:white_check_mark: Hey rowanjacobs! The commit authors and yourself have already signed the CLA.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/168306507

The labels on this github issue will be updated when the story is started.

[davewalter]

Thanks @rowanjacobs