Vault: stringify JSON response for compatibility with KV backend

[Pluies]

Hello, it's me again 👋

Due to changes in #206,
we now need to stringify the response from the backend which currently comes as
an object rather than a JSON string.

Fixes issue such as:

{"level":30,"time":1573653228258,"pid":19,"hostname":"external-secrets-8zq4bq","msg":"running poll on the secret example/example","v":1}
{"level":40,"time":1573653228468,"pid":19,"hostname":"external-secrets-8zq4bq","msg":"Failed to JSON.parse value for 'secrets/data/example/credentials', please verify that your secret value is correctly formatted as JSON. To use plain text secret remove the 'property: apiToken'","v":1}

[Pluies]

cc @Flydiverny

[Flydiverny]

Ah, doh! can you update the test so the build passes ?

[Pluies]

can you update the test so the build passes ?

Eh! My turn to "doh" :)

Done on this PR; and I'm working on tests that will run vaultBackend.getSecretManifestData to test more than the (fairly straightforward) inner loop of _get. I also want to include tests with broken data or failed responses from Vault to test edge cases and ensure they're handled correctly. I'll open a separate PR once this is ready 👍