Add step to prompt about app's authentication type

[nturinski]

The new prompt asks a user for using secrets of managed identity. There have been enough issues with MI that we should offer users the old method of creating even if it's less secure.

Other small changes include:

• The app setting is supposed to be managedidentity apparently (note the lack of a capitial I)
• Documentation/Portal states that you only need Storage Data Blob Contributor role
• Don't prompt for instance memory size or maximum instance count for basic create