fixes ZEN-13394: honor env var SERVICED_MAX_CONTAINER_AGE and --max-cont...

...ainer-age when calling reapContainers()

DEMO1:
```
/home/plu/src/europa/src/golang/bin/serviced -v=1 --master --agent
...
I0806 21:17:14.252714 14145 agent.go:771] reapOldContainersLoop starting
I0806 21:17:14.252738 14145 agent.go:274] will reap stopped containers older than age of 24h0m0s
```

DEMO2:
```
/home/plu/src/europa/src/golang/bin/serviced -v=1 --master --agent --max-container-age 120
…
I0806 21:11:15.225928 09782 agent.go:771] reapOldContainersLoop starting
I0806 21:11:15.225945 09782 agent.go:274] will reap stopped containers older than age of 2m0s
```

DEMO3:
```
root@plu-9:~# SERVICED_MAX_CONTAINER_AGE=240 /home/plu/src/europa/src/golang/bin/serviced -v=1 --master --agent
...
I0806 21:27:05.613669 16964 agent.go:771] reapOldContainersLoop starting
I0806 21:27:05.613679 16964 agent.go:274] will reap stopped containers older than age of 4m0s
```