Bugfix: functest only keeps the last test case's log

Bugfix: functest only keeps the last test case's log

JIRA: DOVETAIL-490

1. Dovetail just keeps the last tempest/defcore/ipv6/bgpvpn test's log file.
2. All log files should be kept, so users and developers can know the reason
   of failed test cases.
3. tempest logs will be stored in directory tempest_logs/
4. defcore logs will be stored in directory defcore_logs/
5. IPv6 logs will be stored in directory ipv6_logs/
6. bgpvpn logs will be stored in directory bgpvpn_logs/
7. All vping logs are in functest.log

Change-Id: I79eae79cb32d65ce1ada3dd6f4c6dfd3945fc512
Signed-off-by: xudan <xudan16@huawei.com>