prepare for testresults.opnfv.org's watchdog

prepare for testresults.opnfv.org's watchdog

Due to the frequent crash in testresults.opnfv.org, watchdog will be introduced
to monitor the status of docker daemon/testapi container/reporting container,
to make timely recovery, to make things clear, some refactors included
in this patch:
1) leverage container name and image name rather than id
2) address three conditions during the update
3) refactor code to be more readable

Change-Id: Ifa7a6fd94db62d532026b5fad33835b0e2abd9af
Signed-off-by: SerenaFeng <feng.xiaowei@zte.com.cn>