Allow display_name for v2 snapshot-update

Allow display_name for v2 snapshot-update

According to the comment in the source code, the snapshot-update of
v2 would be able to allow either "name" or "display_name" parameter.
And if both paramters are specified, "name" parameter is effective
over "display_name" like "description" parameter.
However "display_name" parameter is ignored now, because cinder writes
over empty dict(update_dict['display_name']).

This patch fixes the problem.

Fixes bug #1226398

Change-Id: Icd48556a26b9346f73b7c1b82ffbcdd469c4d28d