Modify ping test threshold

Change the threshold to 5 VM pairs for basic common
OPNFV verification requirements from Dovetail.
Currently, offline test over apex pass:
1. ping subtest1:
   https://gist.github.com/gabrielyuyang/764b7ee636ac5502d5e561a348ae909b
2. ping subtest2:
   https://gist.github.com/gabrielyuyang/af17905a7e2379e870f492ea393c54c0

Working on testig over compass, trying to deploy a compass
first. We could also see the result from compass on CI
after this patch be merged. Results in Dovetial CI will
also be updated then.

Change-Id: If2cfe9988c046d733b30a23de3cd569ed06496e6
Signed-off-by: Yang (Gabriel) Yu <Gabriel.yuyang@huawei.com>
(cherry picked from commit 62feef57918f110d423961a3989f77cf815058c6)