Create two additional waiters AppAvailable and InstanceProfileAvailable

[djcp]

This creates 2 additional waiters:

• OpsWorks AppAvailable, to ensure an app is available before taking subsequent actions
• IAM InstanceProfileAvailable, to let you wait for an instance profile to be available

[trevorrowe]

I pulled these changes locally, and made a few minor changes. Most notably, I renamed the waiters to InstanceProfileExists and AppExists to be consistent with other waiter names. I also added a resource exist waiter for Aws::IAM::InstanceProfile#wait_until_exists and Aws::IAM::InstanceProfile#exists?. The latter is automatically created when there is an exists waiter.

Thanks for the contribution! This should be released shortly.

[djcp]

Thanks!