Make MaxTries configurable

[Romanas Sonkinas]

It's currently hardcoded to 3 attempts, which is not enough in some cases. I.e. we have a heavily used testing account where API throttling happens often. The code in question is this:

in aws/client.go

var retryingTransport = &ResilientTransport{
        Deadline: func() time.Time {
                return time.Now().Add(5 * time.Second)
        },
        DialTimeout: 10 * time.Second,
        MaxTries:    3,
        ShouldRetry: awsRetry,
        Wait:        ExpBackoff,
}