(cloudwatch) Fix UnHealthyHostCount metric of ApplicationELB

The UnHealthyHostCount metric of ApplicationELB for cloudwatch datasource did not work because the capitalization was wrong - it was UnhealthyHostCount instead of UnHealthyHostCount.
Here is the AWS documentation listing the metric name - http://docs.aws.amazon.com/elasticloadbalancing/latest/application/load-balancer-cloudwatch-metrics.html#load-balancer-metrics-alb

pkg/api/cloudwatch/metrics.go

@@ -29,7 +29,7 @@ var customMetricsDimensionsMap map[string]map[string]map[string]*CustomMetricsCa
 func init() {
 	metricsMap = map[string][]string{
 		"AWS/ApiGateway":     {"4XXError", "5XXError", "CacheHitCount", "CacheMissCount", "Count", "IntegrationLatency", "Latency"},
-		"AWS/ApplicationELB": {"ActiveConnectionCount", "ClientTLSNegotiationErrorCount", "HealthyHostCount", "HTTPCode_ELB_4XX_Count", "HTTPCode_ELB_5XX_Count", "HTTPCode_Target_2XX_Count", "HTTPCode_Target_3XX_Count", "HTTPCode_Target_4XX_Count", "HTTPCode_Target_5XX_Count", "NewConnectionCount", "ProcessedBytes", "RejectedConnectionCount", "RequestCount", "TargetConnectionErrorCount", "TargetResponseTime", "TargetTLSNegotiationErrorCount", "UnhealthyHostCount"},
+		"AWS/ApplicationELB": {"ActiveConnectionCount", "ClientTLSNegotiationErrorCount", "HealthyHostCount", "HTTPCode_ELB_4XX_Count", "HTTPCode_ELB_5XX_Count", "HTTPCode_Target_2XX_Count", "HTTPCode_Target_3XX_Count", "HTTPCode_Target_4XX_Count", "HTTPCode_Target_5XX_Count", "NewConnectionCount", "ProcessedBytes", "RejectedConnectionCount", "RequestCount", "TargetConnectionErrorCount", "TargetResponseTime", "TargetTLSNegotiationErrorCount", "UnHealthyHostCount"},
 		"AWS/AutoScaling":    {"GroupMinSize", "GroupMaxSize", "GroupDesiredCapacity", "GroupInServiceInstances", "GroupPendingInstances", "GroupStandbyInstances", "GroupTerminatingInstances", "GroupTotalInstances"},
 		"AWS/Billing":        {"EstimatedCharges"},
 		"AWS/CloudFront":     {"Requests", "BytesDownloaded", "BytesUploaded", "TotalErrorRate", "4xxErrorRate", "5xxErrorRate"},