Add REST API to show availability_zone of instance.

Implement one workitem for bp:show-availability-zone

This show the availability zone and availability zone of its host in
API: GET /servers/details, GET /server/{uuid}.

When create instance if "availability_zone" doesn't specified, the
"availability_zone" of the instance will be None.  But actually the instance
is in a zone which its host is in. So I think show both of them will be fine.

Change-Id: I2d742ee2d291b514bf783fa79293785cca8ea2a5
Merge "Add REST API to show availability_zone of instance."