Correcting 'id' type in compute schema

This patch corrects the type of 'id' in compute schema
/tempest/api_schema/compute/.
Many place 'id' is defined as both 'string' and 'integer' but not all
APIs return 'id' in both format.

For example, servers APIs always return 'server id' as UUID so those
should be defined as 'string' only not {'string', 'integer'} both.
Same applies for other 'id' like in flavor, Image API's response.

Below are the correct type of 'id'-
  Server's id – String
  Image id – String
  Flavor id - String
  key-pair API- > user_id – String
  Agent id – Integer
  Service id - Integer

Partially implements blueprint nova-api-attribute-test

Change-Id: Ifdde2ddd56d600f968d02356aaec0c56e5c3767f
Merge "Correcting 'id' type in compute schema"