s3's attempt strategy is very long for tests

Most of juju-core's test suite is spent waiting for the AttemptStrategy in goamz.s3. On a fast multi-core machine, using a default (all-zero) AttemptStrategy in goamz.s3 makes the overall juju-core test suite run about 3 times faster, and the EC2 provider tests by themselves over 100 times faster.

It would be nice if clients of goamz had access to a capability along the lines of SetAttemptStrategy for their tests. Not necessarily *exactly* that though -- I'm not trying to dictate solutions here, just describing the problem. :-)