Tweak disagreement ratio calcuation
Tweak disagreement ratio calcuation

Divide by the number of +1/+2 votes, not the total number of votes.

Change-Id: Ia77a7a711c7969fcc45679589a8ba2763cd8c119

openstack-infra/reviewstats,cp16net/reviewstats,JohnGarbutt/reviewstats,openstack-infra/reviewstats,JohnGarbutt/reviewstats,cp16net/reviewstats