Add Redis identity test

Run identity tests against a redis DbContext.

@lajones Can you review the redis stuff.  I copied this from Redis.Functional pretty much as is?  I had to disable 2 tests due to key generation not working.
Looks like I already filed a bug: https://github.com/aspnet/EntityFramework/issues/651  And it was closed as fixed, do I need to do something to use this feature?