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 
If we put this first in this method then we can delete all sites safely. I would prefer that then the Guid parse.

The only other thing is that sometimes having the default site is useful so I think this should be something that is configured via a boolean passed into the constructor. I think I'd like to keep the current behaviour as the default.

Do you mind making those changes then I'll merge it in and push it out to NuGet.