Add unique constraint for security groups

- Add unique constraint (project_id, name, deleted)
  for security_groups
- Define new SecurityGroupExists exception
- Drop security_group_exists() in favor of catching
  SecurityGroupExists exception for security_group_create()

Fixes bug #1093458

Blueprint: db-enforce-unique-keys

Change-Id: Id9a1427587cc9250942d0abd9eafc69861352d9b
Merge "Add unique constraint for security groups"