CSIT-1139: Implement parallel test execution

CSIT-1139: Implement parallel test execution

The implementation of parallel test execution in VPP Test Framework.
- VPPTestCase test methods are grouped together
- tests are running in separate processes
- VPP instances spawned by tests are assigned to different cores
- output from these processes is redirected through pipes and printed
out testcase by testcase
- TEST_JOBS env var is used to specify the number of parallel processes
- improved test summary
- a bit of code cleanup

Change-Id: I9ca93904d9fe2c3daf980500c64a8611838ae28c
Signed-off-by: juraj.linkes <juraj.linkes@pantheon.tech>