Don't try to execute _stop commands

These are emitted when the command socket is being shutdown and is
only used to wake the thread. It should not be accepted as a command.

Change-Id: I0e7b30cbe60f5a96daec3697f24b9a97516027e2
diff --git a/zuul/launcher/server.py b/zuul/launcher/server.py
index 74cc2be..bfcc8a4 100644
--- a/zuul/launcher/server.py
+++ b/zuul/launcher/server.py
@@ -280,7 +280,8 @@
         while self._command_running:
             try:
                 command = self.command_socket.get()
-                self.command_map[command]()
+                if command != '_stop':
+                    self.command_map[command]()
             except Exception:
                 self.log.exception("Exception while processing command")