Committer promotion Dan Xu and removal of Leo Wang

Committer promotion Dan Xu and removal of Leo Wang

Contributions Dan Xu:
Dan Xu has been almost exclusively doing the development for the
Dovetail tool, CI integration, and the practical validation of test case
candidates.
https://gerrit.opnfv.org/gerrit/#/q/owner:%22Dan+Xu+%253Cxudan16%2540huawei.com%253E%22

Leo Wang has announced that he is stepping down as committer from the
project. Thank you for your contributions to the project.
https://lists.opnfv.org/pipermail/opnfv-tsc/2018-February/004130.html

Change-Id: I14032657b057bdd3da1f9c3514b3c27e34dbafc0
Signed-off-by: Georg Kunz <georg.kunz@ericsson.com>