Close ssh connections when done with them

Newer paramiko leaks connections if they are not explicitly closed. Add
a finally handler to always close the ssh connections when we are done
with it to avoid leaking these connections.

Change-Id: Ia2e53998d362683a42bda074d82e3a3a75f380b4

gerritlib/gerrit.py

@@ -396,6 +396,7 @@ class Gerrit(object):
         return data
 
     def _ssh(self, command):
+        try:
             client = paramiko.SSHClient()
             client.load_system_host_keys()
             client.set_missing_host_key_policy(paramiko.WarningPolicy())
@@ -415,6 +416,9 @@ class Gerrit(object):
 
             err = stderr.read()
             self.log.debug("SSH received stderr:\n%s" % err)
+        finally:
+            if client:
+                client.close()
         if ret:
             raise Exception("Gerrit error executing %s" % command)
         return (out, err)