bugfix: Graceful shutdown of VM - improvement

bugfix: Graceful shutdown of VM - improvement

Cleanup phase of PVVP scenario sometimes causes server reboot.
Following updates were made to prevent reboots:
* better generic process termination procedure
* ovsdb is terminated after vswitchd termination
* vswitchd is terminated directly instead of parent sudo process
* already running VNFs are terminated in case of failure during
VNF start()

Change-Id: Ic09d60d7bfdea01c84a2685ede3d0316f0d09be7
JIRA: VSPERF-271
Signed-off-by: Martin Klozik <martinx.klozik@intel.com>
Reviewed-by: Maryam Tahhan <maryam.tahhan@intel.com>