Update GRE tunnels syslog configuration

Summary:
D4097 was not complete, so this review ensures that the log file works properly.

Ref T2322

Test Plan:

• logger -t gre-tunnels "hello after salt"
• tail -f /var/log/gre-tunnels.log

Reviewers: dereckson, Duranzed

Reviewed By: dereckson, Duranzed

Subscribers: Duranzed, dereckson

Maniphest Tasks: T2322

Differential Revision: https://devcentral.nasqueron.org/D4101