Ability to have IIS App Pools use special builtin accounts

Added ability to set IIS app pool identity model as NetworkService, LocalService, LocalSystem via "magic strings" passed in as IdentityUsername

I needed to set up a deploy as a NetworkService, but while the docs implied I could send in NetworkService to get this behavior, it was actually setting a SpecificUser with that name.  

Tested on my deploy, and it now works as expected. 

PS: Can't imagine anyone would be trying to use a built in account with the same name, but they can specify ".\NetworkService" if they really want to