Add two new options for the MySQL plugin

This pull request add `Alias` and `ConnectTimeout` options for the MySQL plugin.

`Alias` is meant to be used when your hostname makes less sense. This is at least useful for me when monitoring Amazon RDS and having super long hostnames that doesn't fall into my regular namespace. Skip this option for usual behaviour.

`ConnectTimeout` lets the user configure the time before marking the TCP connection as failed upon connect. Defaults to 0 which is the same as before.

**Example configuration:**

    <Plugin mysql>
        <Database db1>
            Alias "rds.example.com"
            Host "db1.abc1atka1fvc.eu-west-1.rds.amazonaws.com"
            User "collectd"
            ConnectTimeout 5
        </Database>
    </Plugin>