This PR adds the option to set in-memory certificates to the API client instead of requiring the certificate to be stored on disk in a file.
This allows us to define API client TLS options per Consul secret backend in Vault.
Related issue hashicorp/vault#4800
This only affects vault versions >=1.1.1 because the prior code
accidentally relied upon a bug that was fixed in
https://github.com/hashicorp/vault/pull/6505
The existing tests should have caught this, but they were using a
vendored copy of vault version 0.10.3. This fixes the tests by running
an actual copy of vault instead of an in-process copy. This has the
added benefit of changing the dependency on vault to just vault/api.
Also update VaultProvider to use similar SetIntermediate validation code
as the ConsulProvider implementation.
