Add info for Standalone EC2 API to cut access to Nova DB.

This change adds properties to v2.1
os-extended-server-properties and os-extended-volumes nova APIs
in order to expose information which is required for the standalone
EC2 API implementation (resides on stackforge/ec2-api) to use only
public APIs. Right now this information is taken by directly accessing
internal Nova DB.
It adds proposed microversion v2.3.
It implements the spec "Nova changes required for standalone EC2 API
implementation" in this review: https://review.openstack.org/#/c/153636/

APIImpact
Implements: blueprint ec2-api

Change-Id: I850e51e730c82906f68fddb14fb0004d7091eb91

nova/api/openstack/api_version_request.py

@@ -40,6 +40,8 @@ REST_API_VERSION_HISTORY = """REST API Version History:
     * 2.1 - Initial version. Equivalent to v2.0 code
     * 2.2 - Adds (keypair) type parameter for os-keypairs plugin
             Fixes success status code for create/delete a keypair method
+    * 2.3 - Exposes additional os-extended-server-attributes
+            Exposes delete_on_termination for os-extended-volumes
 """
 
 # The minimum and maximum versions of the API supported
@@ -48,7 +50,7 @@ REST_API_VERSION_HISTORY = """REST API Version History:
 # Note(cyeoh): This only applies for the v2.1 API once microversions
 # support is fully merged. It does not affect the V2 API.
 _MIN_API_VERSION = "2.1"
-_MAX_API_VERSION = "2.2"
+_MAX_API_VERSION = "2.3"
 DEFAULT_API_VERSION = _MIN_API_VERSION