Don't read certs from the vault server's local filesystem

[mgaffney]

The ca_cert, client_cert, and client_key configuration parameters are paths to files. These files are read from the local filesystem the vault server is running on. It would be better if the contents of those files were uploaded and stored in vault not read from the local filesystem.

[tyrannosaurus-becks]

There's discussion of this here as well: hashicorp/terraform-provider-vault#704 (comment)