Support to specify docker image for each test case

Support to specify docker image for each test case

1. Currently each type of test cases will use one docker image.
2. For example,
   Functest test cases use opnfv/functest-restapi:euphrates
   Yardstick uses opnfv/yardstick:opnfv-5.1.0
3. It needs to support to specify the docker image for each test case.
4. For example,
   vping test cases use opnfv/functest-smoke:euphrates
   sdnvpn uses opnfv/functest-features:euphrates
5. The benefit of this is that these docker images are well verified
   by Functest CI and from Functest plan, they may not support
   functest-restapi docker image in its future release.

JIRA: DOVETAIL-638

Change-Id: I8a30eea2233aeff809af2e241b6c939194397832
Signed-off-by: xudan <xudan16@huawei.com>