Add setting to define custom MySQL port.

[noplanman]

MySQL credentials array now allows a port parameter to be set:

$mysql_credentials = [
   'host'     => 'localhost',
   'port'     => 3306,        // new optional parameter
   'user'     => 'dbuser',
   'password' => 'dbpass',
   'database' => 'dbname',
];

Fixes #857

[akalongman]

Looks good 👍