dovetail: conditional steps added to fuel-bgpvpn scenario

dovetail: conditional steps added to fuel-bgpvpn scenario

JIRA: DOVETAIL-432

1, only run against fuel os-odl_l2-bgpvpn-ha scenario
this can reduce the CI/pod time resource
dovetail test cases by now (refstack, ha, ipv6, bgpvpn)
2, the conditional steps have been successed tested on compass
  to remove the introduce of
  https://gerrit.opnfv.org/gerrit/#/c/34327/
  conditonal steps look more simple and understandable

Change-Id: I315a789160b8173274b1c1ef916073ced7d5e91b
Signed-off-by: MatthewLi <matthew.lijun@huawei.com>