Add unit tests for processing utils
Add unit tests for processing utils

Added following unit tests
test_percentile_value_none
test_percentile_equal

Change-Id: I772bcfae418722371b7abcd525fcf913aef1e01e
Implements: blueprint improve-unit-test-coverage-rally

gluke77/rally,eonpatapon/rally,amit0701/rally,aforalee/RRally,shdowofdeath/rally,vganapath/rally,group-policy/rally,eayunstack/rally,paboldin/rally,group-policy/rally,vganapath/rally,pyKun/rally,vponomaryov/rally,varunarya10/rally,yeming233/rally,openstack/rally,vponomaryov/rally,pandeyop/rally,amit0701/rally,cernops/rally,gluke77/rally,openstack/rally,varunarya10/rally,openstack/rally,cernops/rally,vganapath/rally,gluke77/rally,eayunstack/rally,yeming233/rally,amit0701/rally,eonpatapon/rally,openstack/rally,eayunstack/rally,paboldin/rally,redhat-openstack/rally,vefimova/rally,afaheem88/rally,gluke77/rally,vganapath/rally,go-bears/rally,aplanas/rally,vefimova/rally,afaheem88/rally,aplanas/rally,paboldin/rally,shdowofdeath/rally,pyKun/rally,pandeyop/rally,redhat-openstack/rally,aforalee/RRally,go-bears/rally,group-policy/rally