Do not fail test case when no vpp pid detected in the tear down

Do not fail test case when no vpp pid detected in the tear down

- currently when no pid (or more pids) detected in the test case
  tear down of successfully run test case not only the WARN message
  is logged but the whole test case is failed

- to change this behaviour but keep test case failure if no vpp pid
  (or more vpp pids) detected in test case setup the low level KW
  "Get VPP PID" behaviour has been changed to return one or more
  pids or None w/o raising an error and check has been moved to
  higher level KW "Save VPP PIDs"

- there was added suite name and test case name to WARN message when
  no pid or more pids detected in the test case tear down

Change-Id: I04f8d81c1ca48a1e4a45bd0e58f00d36d51c6933
Signed-off-by: Jan Gelety <jgelety@cisco.com>