[Improvement] Wait on waitgroup instead of using time.Sleep when retrieving stats for named containers

[bn4t]

Description

Relevant Code: https://github.com/docker/cli/blob/master/cli/command/container/stats.go#L175

Instead of using a fixed time.Sleep(1500 * time.Millisecond) when collecting stats for named containers, just wait on the waitFirst waitgroup instead.