refactor(cookie): Warning message when boot with default Erlang cookie

[zmstone]

$ env EMQX_NODE_COOKIE='' ./_build/emqx/rel/emqx/bin/emqx console
!!!!!!
WARNING: Default (insecure) Erlang cookie is in use.
WARNING: Configure node.cookie in /mnt/code/src/emqx/0/_build/emqx/rel/emqx/etc/emqx.conf or override from environment variable EMQX_NODE__COOKIE
NOTE: Use the same config value for all nodes in the cluster.
!!!!!!
log.file_handlers.default.enable = EMQX_LOG__FILE_HANDLERS__DEFAULT__ENABLE = false
log.console_handler.enable = EMQX_LOG__CONSOLE_HANDLER__ENABLE = true