Fixes Nova API /os-hosts missing element "zone"

Now the HostIndexTemplate of Nova /os-hosts API is missing element "zone",
which will cause the XML output of list hosts missing the zone attribute.

Fixes bug: 1159055

Change-Id: I9940efcfd364c35b00cde4398d531d7ff56dc2b9
Merge "Fixes Nova API /os-hosts missing element "zone""