Set is_public to False by default for volume backed snapshots

This update results in the same behavior that is already implemented
in _create_image where the meta_data for is_public is set to False.

Originally committed by Abel Lopez via puppet-coe-service-patch
and cherry picked from upstream here.  Abel's commit log:

---
commit 17a5e697b380b8051dc2967b2e6a32be83ebec35
Author: Abel Lopez <abelopez@cisco.com>
Date:   Mon Jul 14 10:11:57 2014 -0700

    DE391 - Private snapshots by default

    This addresses DE391 - Taken from Icehouse branch at
    https://github.com/openstack/nova/blob/stable/icehouse/nova/compute/api.py#L
    Snapshots should be Private, this line was overlooked in Havana for
    Volume backed snapshots.

    Change-Id: Iaba3b6441b56a4a93a79441732c9e4b433c05b26
---

Change-Id: Ie0720d5c847871527db7d9477ca6cb05abe7a1a5
Closes-Bug: #1255316
Implements: rally-user-story US4145
Implements: rally-task TA3473
Closes-rally-bug: DE391
Upstream-Review: https://review.openstack.org/#/c/112662/