FoundationDB support added for HashiCorp Vault

With its recent 0.10.4 release, HashiCorp Vault has added a FoundationDB physical backend. This feature adds stateful storage of secrets into of a distributed and highly-available backend. Additionally, this backend implements Vault's transactional interface, needed for some specialized features.

The code powering this new Vault feature is contained in the original pull request #4900, along with configuration information in the Vault documentation. The adapter uses FoundationDB's Go bindings.