dovetail: add a suitable job used for vpn test case validation in CI

dovetail: add a suitable job used for vpn test case validation in CI

JIRA: DOVETAIL-424

the principle is
1) not disturb the original work flow
2) after vpn testcases integrated into dovetail, i.e, CVP tool,
   there is a need to find a POD has bgpvpn feature which is
   suitable to validate bgp vpn test cases in proposed_tests.
3) only add dovetail proposed_tests into bgpvpn scenario deploy-test loop
   to reduce unnecessary time consuming.
4) further, should consider to add dovetail job in os-nosdn-nofeature loop

Change-Id: Iad6ae1e44470c8e9d49a25980d292ba3871bd6a4
Signed-off-by: MatthewLi <matthew.lijun@huawei.com>