Convert v3 server SchedulerHints plugin to v2.1

This patch converts v3 server SchedulerHints 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.

Note- to make V2.1 SchedulerHints extension work in V3 framework,
an extra parameter (request body) is being passed in server_create
function of each extensions.

Partially implements blueprint v2-on-v3-api

Change-Id: I8a0d50efcda6fcd12971a41505127de5987eec18
Merge "Convert v3 server SchedulerHints plugin to v2.1"