Fix setting of GIT_SSH for timer merge jobs

Problem is that merge jobs triggered by timer don't supply
"refspec" event attribute, and, naturally, nothing is merged
but the repo is updated.
But, in such case, no connection info is passed to merger,
and GIT_SSH is not set.
So, the merge job will fail (perhaps, until some
other change events with "refspec" defined will
make this merger to set GIT_SSH properly)

Change-Id: I6dd96ae0bdcdf4c3db4c931a8c6ef7d5edfc1257

tests/fixtures/zuul-connections-same-gerrit.conf

@@ -26,13 +26,13 @@ logserver_prefix=http://logs.example.org/server.app/
 driver=gerrit
 server=review.example.com
 user=jenkins
-sshkey=none
+sshkey=fake_id_rsa1
 
 [connection alt_voting_gerrit]
 driver=gerrit
 server=review.example.com
 user=civoter
-sshkey=none
+sshkey=fake_id_rsa2
 
 [connection outgoing_smtp]
 driver=smtp