Change 'image_ref'/'flavor_ref' into v2 style for servers

This patch changes 'image_ref'/'flavor_ref' to 'imageRef'/'flavorRef'
that same with v2 api. And also change the related unittest.
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: Ic17334e0a9de89488d5e0c8b75950027d51a3cd4
Merge "Change 'image_ref'/'flavor_ref' into v2 style for servers"