provisioner.go

When retrieving data from a vault using `out` to store the data in a
file resulted is random characters being returned and logged. These
characters could generate a traceback print from Ansible's logger,
without breaking the script.

The reason for that is that the result from `vault_retrive` was being
processed when it was not needed, and data was beeing returned, when
it shouldn't.

This patch fixes this behavior by supressing the return data when `data`
is not available, and only raising an error if it should be available.