Port security groups extension to v3 API Part 2

This patch contains the changes required to adapt the
security groups extension and the corresponding unittests
to the v3 framework.

In v3, the management of security groups will be handled
by the networking service API. However, the dis/association
of security groups with instances still needs to be handled
by Nova. This change removes the security group management
code and ports the dis/association code to v3.

Partially implements blueprint nova-v3-api

Change-Id: I617eee5660447bc61518b89eed73ce394f7435a2
Merge "Port security groups extension to v3 API Part 2"