Adding Longevity Test for clustering data store

Usage example : pybot -v IP1:10.195.128.18 -v PORT:8181 -v IP2:10.195.128.35 -v IP3:10.195.128.19 -v DURATION:500 010__longevity.txt

Change-Id: I1d40e366bb59b14822dd6fa17a42146c700684ac
Signed-off-by: Harman Singh <harmasin@cisco.com>