make 'server' as 'required' in server update schema

This patch make the 'server' attribute as 'required' in
server update schema (base_update_server).

This attribute is not optional in server's update API response.

Partially implements blueprint nova-api-attribute-test

Change-Id: Ibec518910597fc9e03a69d1d2bc94e719cfab0f9
Merge "make 'server' as 'required' in server update schema"