Improve secrets

[jesusfcr]

This PR allows vulcan to use existing secrets.

Those secrets could be previously been generated manually, with SealedSecrets, with external-secrets, ...

Provided we have a secret for the db:

kubectl create secret generic db --from-literal=password=<your_password>

We can use that secret:

vulndb:
  db:
    existingSecret: db
    existingSecretPasswordKey: password