Add destructor for SshAgent

There is no indication this is necessary (it seems all code paths
do result in stop() being called), but just in case, add a destructor
to SshAgent to make sure we don't leave an ssh-agent process running
beyond the end of a job.

Change-Id: I10172574d0844e7181a82ae586121e6ba5e3d6ff

zuul/executor/server.py

@@ -260,6 +260,16 @@ class SshAgent(object):
             self.log.debug('Sent SIGTERM to SSH Agent, {}'.format(self.env))
             self.env = {}
 
+    def __del__(self):
+        try:
+            self.stop()
+        except Exception:
+            self.log.exception('Exception in SshAgent destructor')
+        try:
+            super().__del__(self)
+        except AttributeError:
+            pass
+
     def add(self, key_path):
         env = os.environ.copy()
         env.update(self.env)