Cloudfront waiters

Fixes: https://github.com/aws/aws-cli/issues/1059

There were two changes made in this PR:

1) Make the JMESPath success path for the waiter take into account the outer element in the cloudfront response.  Without the outer element included, you get a bunch of max retries exceeded because the JMESPath query is not using the correct path to search for the success state.

2) Doubled the max retry attempts. I tested it for a single object invalidation, and with the current configuration, the max retries were consistently reached. I was estimating it took around 15 minutes to complete the invalidation, but the current configuration would wind up waiting for 10 minutes. By doubling the retry attempts, it bumped this to 20 minutes. 

cc @jamesls @danielgtaylor 
Were you seeing cache issues before you added this?  Line 522 we're not passing in a cache to the loader so it should be recreating a brand new cache each test.