Print SIGTERM logging to debug

Sending SIGTERM to the ssh-agent is a standard part of shutting down the
executor. It stands out it logs when everything else is at debug, but a
TERM signal is more important. This is not something that needs looked
at so should just be debug.

Change-Id: I31f15cc24bb185a0f95ac9d1c30ba852fd5cbd53
diff --git a/zuul/executor/server.py b/zuul/executor/server.py
index 3daafc7..b7c2f7f 100644
--- a/zuul/executor/server.py
+++ b/zuul/executor/server.py
@@ -203,7 +203,7 @@
             except OSError:
                 self.log.exception(
                     'Problem sending SIGTERM to agent {}'.format(self.env))
-            self.log.info('Sent SIGTERM to SSH Agent, {}'.format(self.env))
+            self.log.debug('Sent SIGTERM to SSH Agent, {}'.format(self.env))
             self.env = {}
 
     def add(self, key_path):