collector/cluster_health.go:Fix golint issues
collector/cluster_health.go:Fix golint issues

See,
$ gometalinter --vendor --disable-all --enable=golint collector/cluster_health.go
collector/cluster_health.go:36:6:warning: exported type ClusterHealth should have comment or be unexported (golint)
collector/cluster_health.go:48:1:warning: exported function NewClusterHealth should have comment or be unexported (golint)
collector/cluster_health.go:212:1:warning: exported method ClusterHealth.Describe should have comment or be unexported (golint)
collector/cluster_health.go:247:1:warning: exported method ClusterHealth.Collect should have comment or be unexported (golint)

ewr/elasticsearch_exporter,justwatchcom/elasticsearch_exporter