Add external storage

[poblin-orange]

As of v1, terraform state can only be stored in bosh external disk (/var/vcap/store). This requires a persistent disk, and additional backup mechanism.

We should offer some remote terraform backend support.

[gberche-orange]

What about a new job property to let release users specify their own backend, e.g.

https://www.terraform.io/docs/backends/types/s3.html

terraform {
  backend "s3" {
    bucket = "mybucket"
    key    = "path/to/my/key"
    region = "us-east-1"
  }
}