Add empty flavor object info in server api-ref

In case flavor information is not present in sysmeta then
GET/PUT server response can have flavor as empty object.

ref- http://git.openstack.org/cgit/openstack/nova/tree/nova/api/openstack/compute/views/servers.py#n252

Having that mentioned in api-ref will be nice to understand it clearly.
Closes-Bug: #1677999

Change-Id: I350dc33df0cb03eb86df1f142e58ae34b02a1334

api-ref/source/parameters.yaml

@@ -2189,6 +2189,14 @@ flavor_rxtx_factor_in:
   in: body
   required: false
   type: float
+flavor_server:
+  description: |
+    The ID and links for the flavor for your server instance. A flavor is a combination
+    of memory, disk size, and CPUs. This can be empty object in case flavor information
+    is not present in the system.
+  in: body
+  required: true
+  type: object
 flavor_swap:
   description: |
     The size of a dedicated swap disk that will be allocated, in

api-ref/source/servers.inc

@@ -534,7 +534,7 @@ Response
   - server: server
   - addresses: addresses
   - created: created
-  - flavor: flavor
+  - flavor: flavor_server
   - hostId: hostId
   - id: server_id
   - image: image
@@ -620,7 +620,7 @@ Response
   - server: server
   - addresses: addresses
   - created: created
-  - flavor: flavor
+  - flavor: flavor_server
   - hostId: hostId
   - id: server_id
   - image: image
@@ -716,7 +716,7 @@ Response
   - server: server
   - addresses: addresses
   - created: created
-  - flavor: flavor
+  - flavor: flavor_server
   - hostId: hostId
   - id: server_id
   - image: image