json: use top-level sensor-name if provided.

json: use top-level sensor-name if provided.

Currently the default configuration file contains a "sensor-name"
at the root of the configuration file, however, eve-log will only
use it if its specified under eve-log.

Now we will look for it at the eve-log, if present we'll use it
but log a deprecation warning, if its not present we'll look
for sensor-name at the root of the configuration.