Ambari REST API: Rename host-groups in cluster create to host_groups for consistency across api

The host group field name is inconsistent across the blueprint and the cluster create body.
In blueprints it is "host_groups" and in the cluster create call it is "host-groups".
Change the field name in the cluster create call to "host_groups" as the rest of the api uses underscore.

{
  "blueprint" : "single-node-test",
  "host_groups" :[
    { 
      "name" : "host_group_1",  
      "hosts" : [          
        { 
          "fqdn" : "test.novalocal"
        }
      ]
    }
  ]
}