Use functest clean module to do the sdnvpn resources cleanup

Use functest clean module to do the sdnvpn resources cleanup

1. If there throw an exception during the test of one sdnvpn test case,
   it will not do the clean.
2. Then there are many resources left, such as network, subnet, server...
3. This is a bug of sdnvpn see JIRA: SDNVPN-184
4. The patch for fixing this bug https://gerrit.opnfv.org/gerrit/#/c/44777/
5. Currently, use functest module "functest openstack clean" to clean the SUT.

JIRA: DOVETAIL-524

Change-Id: I629fc6fdc9e4c95960f6122358ff219a467d441b
Signed-off-by: xudan <xudan16@huawei.com>