dovetail tool: change function pull_image

dovetail tool: change function pull_image

JIRA: DOVETAIL-378

1. test_runner checks the result of cmd "pull image". If fail to pull,
   then break the running.
2. pull_image will return None when there are errors, otherwise return image name.
3. if there is an old image named the same as the new one, the old one's tag
   will be changed into <none>. In this case, the old image needs to be removed.
4. if there are some containers using this old image, the remove will be skipped
   rather than logging error.

Change-Id: I940cb18911112d5e13f3c14f87ddbdbaced3538a
Signed-off-by: xudan <xudan16@huawei.com>