Change v3 security_groups API to v2.1

This patch changes v3 security_groups API to v2.1 and makes v2 unit tests
share between v2 and v2.1.
This patch doesn't revert /os-security-groups, /os-security-group-rules
and /servers/server_id/os-security-groups because these features are
implemented in Networking API. On v2.1 API, we decide we don't need to
implement proxying features.

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: I82660fd514d041bdd1fad0e689fa4fe90d441c47
Merge "Change v3 security_groups API to v2.1"