ec2_snapshot idempotency feature + some other changes

Added a snapshot_max_age parameter, that allows you to check for an existing snapshot to the volume and only create a new one if it's older than a given age.

"Side" changes:
- Added snapshot_max_age, wait and wait_timeout to the docs
- Made the default value of wait to be false. This restores the behavior to what it was before #6697.