Port assisted-volume-snapshots extension to v2.1

This patch ports assisted-volume-snapshots to v2.1 and make v2 and v2.1
share unit test cases.
This patch addes a schema to do the input validation for snapshots_create
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: I5b7be1de8ac2628a287897dcc5ca0eaf7a8957a2
Merge "Port assisted-volume-snapshots extension to v2.1"