Refactor imports and invocation of main function

Refactor imports and invocation of main function

In all testcases there are some inconcistencies of
import statements. Additionally the main function
of each testcase should be called through
sys.exit(main())

JIRA: SDNVPN-173

Change-Id: I6fdf0dc6598ca4789b6de6b50d1b8b85a8de923c
Signed-off-by: panageo2 <panageo@intracom-telecom.com>