Close command socket after sending stop

After sending the stop signal to the command socket it should be
closed explicitly. Otherwise a resource warning [1] is emitted to
stderr.

[1]: Resource warning:

(...)/zuul/executor/server.py:644: ResourceWarning: unclosed <socket.socket fd=24, family=AddressFamily.AF_UNIX, type=SocketKind.SOCK_STREAM, proto=0, raddr=(...)/executor.socket>
  self.command_socket.stop()

Change-Id: I0e85ea9f5021e329abf825afbd8e29d134c08669
1 file changed