Fixed the inconsistent VCPU-Hours summary

This Period's VCPU-Hours" is retrieved from 
usage.summary.vcpu_hours.However, this is not
the actual total VCPU hours, and usage.summary.vcpus
should be used instead.

Change-Id: Ib65ab6048037ed2b4a1e4f510587b02a858f1f4f
closes-bug:#1407568

horizon/templates/horizon/common/_usage_summary.html

@@ -19,7 +19,7 @@ <h3>{% trans "Select a period of time to query its usage:" %}</h3>
   <p id="activity">
     <span><strong>{% trans "Active Instances:" %}</strong> {{ usage.summary.instances|default:'0' }}</span>
     <span><strong>{% trans "Active RAM:" %}</strong> {{ usage.summary.memory_mb|mbformat|default:'0' }}</span>
-    <span><strong>{% trans "This Period's VCPU-Hours:" %}</strong> {{ usage.summary.vcpu_hours|floatformat:2|default:'0' }}</span>
+    <span><strong>{% trans "This Period's VCPU-Hours:" %}</strong> {{ usage.summary.vcpus|floatformat:2|default:'0' }}</span>
     <span><strong>{% trans "This Period's GB-Hours:" %}</strong> {{ usage.summary.disk_gb_hours|floatformat:2|default:'0' }}</span>
   </p>
 </div>