"stats not implemented for rkt" warning should only be emitted once

[doherty]

If filing a bug please include the following:

Nomad version

0.4.1

Operating system and Environment details

linux w/ rkt

Issue

Nomad spews the following log line:

    2016/11/12 19:09:58.484502 [WARN] client: error fetching stats of task go-kit-demo: stats not implemented for rkt

This is not useful; it should only be logged once. (use sync.Once or a helper in your log package)

Reproduction steps

Run Nomad on a system set up with rkt. Submit a job using the rkt driver. Observe the message shown above being logged every 1s.

@diptanu added this log line in 1cae57a

[doherty]

Correction: logs once per second per task/replica

[github-actions]

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