Node logs are duplicated in syslog

Steps to Reproduce:

1. Setup the pool with logLevel=0.
2. Run load test on this pool.

Actual Results:
On nodes with ~7Gb memory, syslog grows up to 6Gb after writing about 4000 transactions, which results "No space left on device" error.

Expected Results:
All node logs should not be duplicated in syslog.

Additional Information:
As workaround, add StandardOutput=null to /etc/systemd/system/indy-node.service