boto3 move missed a few things

[phobologic]

It looks like the recent boto3 move (in #174) wasn't totally complete - @troyready fixed one of the hooks, but I mistakenly didn't look at the rest of the hooks. It looks like the boto2 library is still used in these places:

stacker/hooks/iam.py
stacker/hooks/ecs.py
stacker/hooks/route53.py
stacker/providers/aws/default.py
stacker/util.py

This was found due to tests failing in #185

(CC: @acmcelwee - if you have a chance to look at it, I'll try to get to it soon otherwise)