Convert v3 server metadata plugin to v2.1

This patch converts v3 server metadata plugin 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: I619a7323f515234d15000af356277dbf3d91231f
Merge "Convert v3 server metadata plugin to v2.1"