Set timeout of Dovetail jobs to be 240 and don't remove the stable images

Set timeout of Dovetail jobs to be 240 and don't remove the stable images

1. Jobs on Compass are always built time out and aborted.
2. Dovetail needs about 2 hours for all the test cases.
3. However, due to the slow internet speed, it will take a long time to pull
   releng source code and dovetail docker image.
4. Extend the time to 240 minutes.
5. Don't remove the stable docker images, so it don't need to pull every time.

JIRA: DOVETAIL-491

Change-Id: I9c7255a89eebf5770c0854224cd65e7413916078
Signed-off-by: xudan <xudan16@huawei.com>