Add Resources for Transit Secrets Backend

[jtcressy]

Hi all,

I wanted a structured way to create transit backends and encryption keys via Terraform, so I went ahead and implemented it for us all.

This PR implements two new resources:

• vault_transit_secret_backend
• vault_transit_secret_backend_key

The transit secret backend is fairly trivial and primitive, since it's simply wrapping vault_mount by setting type = "transit". However I wanted to control the keys within the mount so it seemed natural for there to be resource for the backend.

There is more that could be done on top of this, as I have an idea to implement some new data sources around transit:

• Data sources for encrypting/decrypting/signing/verifying some input value for use elsewhere in terraform (similar to how data.template_file is used)
• Data sources for exporting existing keys, both public and private if applicable.

Checklist:

• Tests cases created & pass
• Documentation created/updated

Acceptance test output:

=== RUN   TestTransitSecretBackendKey_basic
--- PASS: TestTransitSecretBackendKey_basic (0.56s)
=== RUN   TestTransitSecretBackendKey_rsa4096
--- PASS: TestTransitSecretBackendKey_rsa4096 (2.19s)
=== RUN   TestTransitSecretBackendKey_import
--- PASS: TestTransitSecretBackendKey_import (0.33s)
=== RUN   TestTransitSecretBackend_basic
--- PASS: TestTransitSecretBackend_basic (0.39s)
=== RUN   TestTransitSecretBackend_import
--- PASS: TestTransitSecretBackend_import (0.32s)
PASS
ok  	github.com/terraform-providers/terraform-provider-vault/vault	3.986s

Process finished with exit code 0

[jtcressy]

Howdy, @tyrannosaurus-becks! I'm back with more commits. Let me know if there's any problems with this PR so I can fix them.

[tyrannosaurus-becks]

The code, tests, and docs for this look very good. I'm thinking we should strip out the resource marked as having an unknown endpoint, but I'm flexible on that, so I'm looking forward to hearing your thoughts.

[tyrannosaurus-becks]

Thank you! Also, I saw this test failure appeared unrelated, so I did a test build after merging master and the test succeeded.