Allow to specify in which port MySQL is running when creating a new rb site. Currently if database hostname is used (instead of socket) the port option is not working correctly. There is a comment in the code mentioning that in that when MySQL is running in a non default port db_hostname can be specified with hostname:port format. That doesn't seem to work. I have added the new --db_port option instead.

Tested with default and not default ports for MySQL. With this option the settings_local.py file is created correctly and the databases are created.