fixes ZEN-13029: put parseLog warnings into warnings.log in the exported...

... tgz

this feature is needed before additional troubleshooting of https://jira.zenoss.com/browse/ZEN-12806

DEMO1 - when there are no parse warnings:
```
# plu@plu-9: serviced log export
I0725 17:23:22.113158 07068 logs.go:149] Starting part 1 of 3: process logstash elasticsearch results using temporary dir: /tmp/serviced-log-export-440586749
I0725 17:23:22.127860 07068 logs.go:217] Starting part 2 of 3: sort output files
I0725 17:23:22.136321 07068 logs.go:255] Starting part 3 of 3: generate tar file: /home/plu/src/europa/src/golang/src/github.com/control-center/serviced/serviced-log-export.tgz
```

DEMO2 - when there are parse warnings:
```
# plu@plu-9: serviced log export
I0725 17:20:44.568955 06374 logs.go:149] Starting part 1 of 3: process logstash elasticsearch results using temporary dir: /tmp/serviced-log-export-249754172
I0725 17:20:44.595071 06374 logs.go:218] Starting part 2 of 3: sort output files
I0725 17:20:44.605151 06374 logs.go:256] Starting part 3 of 3: generate tar file: /home/plu/src/europa/src/golang/src/github.com/control-center/serviced/serviced-log-export.tgz
W0725 17:20:44.609587 06374 logs.go:264] warnings for log parse are included in the tar file as: serviced-log-export-249754172/warnings.log
# plu@plu-9: tar -xzf serviced-log-export.tgz
~/src/europa/src/golang/src/github.com/control-center/serviced
# plu@plu-9: ls -l serviced-log-export-249754172
total 20
-rw-rw-r-- 1 plu plu 3200 Jul 25 17:20 000.log
-rw-rw-r-- 1 plu plu 4662 Jul 25 17:20 000.log.tmp
-rw-r--r-- 1 plu plu   97 Jul 25 17:20 index.txt
-rw-rw-r-- 1 plu plu  330 Jul 25 17:20 warnings.log
```