utils: copy AttemptStrategy from goamz/aws.

utils: copy AttemptStrategy from goamz/aws.

The aws code, originally derived from the juju-core code,
allows for some neater ways of writing attempt-based code.
In particular it allows us to declare within the loop any variables retrieved
during the attempt.

	for attempt := attempts.Start(); attempt.Next(); {
		x, err := doSomething()
		if shouldRetry(err) && attempt.HasNext() {
			continue
		}
		if err != nil {
			return err
		}
		doSomethingWith(x)
	}

https://code.launchpad.net/~rogpeppe/juju-core/346-crossport-attempt/+merge/176241

(do not edit description out of merge proposal)

[rog, 2013-07-22 16:34:46]

Please take a look.

[mue, 2013-07-23 10:22:15]

LGTM

[gz, 2013-07-23 10:26:11]

LGTM. I'd actually quite like your example usage in the merge proposal as a
comment at the top of the file.

[rog, 2013-07-23 11:09:59]

Please take a look.

[rog, 2013-07-23 11:10:11]

On 23 July 2013 11:26,  <martin.packman@canonical.com> wrote:
> LGTM. I'd actually quite like your example usage in the merge proposal
> as a comment at the top of the file.

I added it as an example function.