Add test coverage

[gberche-orange]

Likely including acceptance tests using the TF acceptance testing framework documented at https://github.com/hashicorp/terraform/blob/master/.github/CONTRIBUTING.md#writing-acceptance-tests, until this appears explictly into https://www.terraform.io/docs/extend/index.html

The CI environment might need to leverage BBL to span an up to date credhub server and access it see related doc

Alternatively, the CI environment could be shared with the one from https://github.com/mevansam/terraform-provider-cf. @mevansam would you be ok with this ?

[ArthurHlt]

Sure it needs acceptance test.

Spawning from bbl is maybe too much as it needs a postgres database + running credhub jar and maybe add uaa which is a just jar too.

[ArthurHlt]

Docker compose ready for acceptance test, see https://github.com/orange-cloudfoundry/credhub-docker