Change v3 attach_interface to v2.1

This patch changes v3 attach_interface API to v2.1 and makes v2
unit tests share between v2 and v2.1.

The differences between v2 and v3 are described on the wiki page
https://wiki.openstack.org/wiki/NovaAPIv2tov3.

Partially implements blueprint v2-on-v3-api

Change-Id: If18676604edc8fea41d7888eb6546245163e91c2
Merge "Change v3 attach_interface to v2.1"