Update to Websites version 2

Add get website instances command.
Update switch slots command with new api.

For the switch command, I've added a second slot so you need to specify the two slots you want to switch (the first one used to always be production).
If you have only 2 slots you don't have to specify any slot as its implicit.

Similar to: https://github.com/Azure/azure-sdk-tools/pull/2564