Add "links" in the response of "nova show" for a down-cell instance

The down-cell microversion 2.69 just recently merged and it returns
links in the response for GET /servers/detail and GET /servers but not
for GET /servers/{server_id} which was an oversight because that API
returns links normally.

We should include the links key in the 'nova show' case as well and this
patch does exactly that.

Typically this would require a microversion change but given the code
merged recently and is not yet released we are just fixing this
oversight through this patch without a microversion bump.

Closes-Bug: #1818131
Change-Id: I2ce03df994f59c37b5ce3102c4e7165d17701798

api-guide/source/down_cells.rst

@@ -240,6 +240,7 @@ behavior are described below:
   - availability_zone: The availability_zone of the server if it was specified
     during during boot time and "UNKNOWN" otherwise.
   - power_state: Its value will be 0 (``NOSTATE``).
+  - links: Links to the servers in question.
   - server_groups: The UUIDs of the server groups to which the server belongs.
     Currently this can contain at most one entry. Note that this key will be in
     the response only from the "2.71" microversion.
@@ -275,6 +276,16 @@ behavior are described below:
                  },
                  "OS-EXT-AZ:availability_zone": "geneva",
                  "OS-EXT-STS:power_state": 0,
+                 "links": [
+                       {
+                           "href": "http://openstack.example.com/v2.1/6f70656e737461636b20342065766572/servers/bcc6c6dd-3d0a-4633-9586-60878fd68edb",
+                           "rel": "self"
+                       },
+                       {
+                           "href": "http://openstack.example.com/6f70656e737461636b20342065766572/servers/bcc6c6dd-3d0a-4633-9586-60878fd68edb",
+                           "rel": "bookmark"
+                       }
+                 ],
                  "server_groups": ["0fd77252-4eef-4ec4-ae9b-e05dfc98aeac"]
              }
          ]