Add backend configs for terraform remote state storage

[sgibson91]

This PR adds a backend config files for terraform that points to various buckets stored in Google projects. This is particularly useful when working on projects we don't have access to via our 2i2c accounts (e.g., Columbia/Pangeo) as we avoid having to work with multiple authentications.

To use a backend file, run:

terraform init -backend-config=backends/NAME_OF_FILE.hcl

Our default remote storage can be configured by

terraform init -backend-config=backends/default-backend.hcl

Remote storage for Pangeo can be configured by

terraform init -backend-config=backends/pangeo-backend.hcl

The -reconfigure option may be required.

fixes #560

[yuvipanda]

This is awesome!

Can you add a blurb to the docs too?

[sgibson91]

Yes, will do! I'll also add the config for our default backend as discussed in Slack tomorrow :)

[sgibson91]

@2i2c-org/tech-team can we all check this PR please? It changes how we initialise terraform so we can dynamically choose backends for terraform state storage

[yuvipanda]

Thanks a lot, @sgibson91! This is sweeeet!

[damianavila]

This is great @sgibson91, thanks!!