Support pagination in describeRackStacks

[thieman]

We're hitting a bug where convox uninstall is returning the following error message to us:

ERROR: can not find rack named (our rack name)
Are you authenticating with the correct AWS account?
See AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY, and ~/.aws/credentials

Pretty sure my credentials are fine as I just used the same command and credentials to successfully uninstall a different rack in our account.

I think what's happening here is we're using DescribeStacks directly but making no effort to consume multiple pages of information. I suspect the rack in question is not included in the first page of results and causes our error. This PR moves us from using DescribeStacks directly to instead use DescribeStacksPages which handles consuming the pages for us.

One note: I think the scoping of apps, rack, and resources is fine but I'm a bit nervous about them as I've been bitten by scoping before. This function also does not seem to be covered directly by any tests (though something calling it may be, I'm not sure).

We've had this problem before: #1385. It may be worth a wider audit of our DescribeStacks usage.

[ddollar]

This will be handled by the CLI change coming in #2739