Netbilling: Support new site_tag credential

For merchants sharing a single account on multiple sites

Netbilling reached out to us:

> I'm writing from Netbilling, a payment gateway that Shopify is integrated with. I'd like to see about having an additional field added to the Netbilling config in the Shopify admin to send with transactions.
> 
> Currently, you can only configure our "account_id" parameter, but "site_tag" should be another field the merchant can set. We use this to associate the transaction with a particular website in the client's account, as they can have more than one. 
> 
> It would be a very small change, so I hope you'll consider adding it. Documentation for that particular field is found in our API spec here:
> 
> http://secure.netbilling.com/public/docs/merchant/public/directmode/directmode3protocol.html#site_tag


Why this new parameter is required:

> As the majority of our merchants process transactions for multiple websites, the “site tag” is used to distinguish which website a transaction is for. There are also a variety of things that are configured at the sitetag level, for example: customizing email receipts, determining which merchant account should be used to process a transaction, and adjusting our “Fraud Defense” filters (various tools we offer to help prevent fraudulent transactions). It’s also used in many of our reports.
> 
> A client of ours is currently processing transactions for multiple websites, but wants to use Shopify for product sales on another site of theirs. The two problems in particular they’re facing, are that they need to use a specific merchant account for the Shopify transactions, and they need to use our reports to separate these transactions out for accounting purposes. Sending this parameter with the transaction is the remedy.

After some follow up they mentioned it's okay to have this new credential be optional.

Support Case: https://shopify.desk.com/agent/case/392085

Related updates for Shopify core and cardserver are being prepared

Please review @jduff