Add description to created change-sets

[bittrance]

Problem: when I get "Stack operation already in progress" I don't know what to do. It would be useful to get some additional info, e.g. who is performing the update and when it was started. While we can't force AWS to provide this via the UI, at least we can make sure Cuffsert is a good citizen and adds some info.

Solution: When creating a change-set, we should call STS get-caller-identity so we can add the ARN to a change-set description identifying the calling user. Similarly, we should present some details about the change-set that is blocking an update rather than just say "Stack operation already in progress". In order for this not to slow down operation, we should probably request list-change-sets in parallel to describe-stacks.