ADDED: PurgeAllDefaultSites() method to IisEnvironment

Not so much a pull request but more of a request for review. 

I've tested it a couple of times and it works although I had to move the purge to the RoleStart() instead of the Run(). Maybe assuming to delete all AppPools and associated websites with a AppPool.Name == Guid is a bit fragile.

I also considered not doing this at all and using a Octopus Powershell stage but I wasn't sure if a Role Recycle would just but the site back.

Let me know what you like ?

John