Reduce number of boskos clusters (#2386)
Reduce number of boskos clusters (#2386)

Now that we have removed 1.3 branch support, there is only one periodic
job that needs boskos, so drop the number of clusters down. we were also
requesting 2 clusters per request, when we only need one.
istio/test-infra,istio/test-infra,istio/test-infra,istio/test-infra