Have clustername returned by all vcenter discovery rules

[JasonEde]

We're finding that as we're adding more vcenters into zabbix it's getting harder to keep track of all the created items. Whilst the esx.discovery returns the cluster name so we can create a host prototype for the cluster HV's none of the others do ({#CLUSTER} is always datastore for the datastore discovery). If {#CLUSTER} or an equivalent was returned for VM's and datastores then we could use that for group prototypes making it significantly easier to keep track of the items.