Allow overriding resource name prefixes

[captainhook]

Really pleased to see a tf module and great job done on aligning with Microsoft recommended prefixes for the resources, e.g. st, plan, log, func

I've implemented Azure naming standards in quite a few businesses now and although the Microsoft recommended naming standard is used, for globally unique resources like storage accounts businesses sometimes use a customised prefix, e.g. cnst, where cn is a 'company name' abbreviation. This grants valuable flexibility to globally unique resource naming.

Would you consider adding an override for the resource name prefixes? Or would you suggest a fork?