Metric::Target capture_cloud_targets rewrite

- only bring back vms that are on (when availability_zone.nil?)
- only bring back vms that have an ems

https://bugzilla.redhat.com/show_bug.cgi?id=1332579
https://bugzilla.redhat.com/show_bug.cgi?id=1331803

app/models/metric/targets.rb

@@ -33,6 +33,7 @@ def self.capture_cloud_targets(zone, options = {})
     # 2) need to find all the VMs that may not be in an availability zone
     # 3) cloudy clusters?
     targets = []
+    return [] if options[:exclude_vms]
 
     includes = {:availability_zones => :tags}
     MiqPreloader.preload(zone.ems_clouds, includes)