Migrate webhook code to the new operator-sdk framework

[don-milos]

The new operator-sdk framework implements the webhook server that we currently have implemented in our own custom code. Migrate the current webhook code to the controllers and use the new framework, i.e. mgr.Add(hookServer) and hookServer.Register(), and remove our custom webhook server code.

[joel-bluedata]

I'm going to take this off of the 0.3 milestone unless some additional motivation for the change pops up.