Convert EC2 API to use nova objects

This blueprint covers updating EC2 API and related functions to use the
Nova object model for all database interaction, like implementation in
compute manager & nova-network now.

Change-Id: Ic00a6e2091115a31748f9b48fc46363f1bba925c
Implements: blueprint ec2-api-objects
Merge "Convert EC2 API to use nova objects"