Don't generate tar file when run without pushing results to DB

Don't generate tar file when run without pushing results to DB

1. Dovetail generate a tar file for uploading results to CVP web portal.
2. If users don't run with option --report, it shouldn't generate the tar file.
   Because users can not upload results to CVP if they don't run with --report.
3. If it generates the tar file when user run dovetail without --report,
   the tar file is invalid because it lacks of results.json.

JIRA: DOVETAIL-511

Change-Id: I4d21f21f0b278962564d810369df03100b38f003
Signed-off-by: xudan <xudan16@huawei.com>