paths: Modify algorithm for PATHS verification

paths: Modify algorithm for PATHS verification

VSPERF verifies if PATHS to the tools (vSwitch, VNF, etc.)
are valid. This check was modified to take into the account
tools required by selected testcase. It will help to avoid
runtime errors triggered by missing tools, which
are not essential for testcase execution.

Change-Id: Ifcfea983cad1bdbc8c1f08c7a0281001005410df
Signed-off-by: Martin Klozik <martinx.klozik@intel.com>
Reviewed-by: Al Morton <acmorton@att.com>
Reviewed-by: Christian Trautman <ctrautma@redhat.com>
Reviewed-by: Sridhar Rao <sridhar.rao@spirent.com>
Reviewed-by: Trevor Cooper <trevor.cooper@intel.com>