HashRing does not handle invalid inputs

[aeva black]

The HashRing class will fail with

  TypeError: must be string or buffer, not None

if passing None to __init__ or to get_hosts.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/70267

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/70267
Committed: https://git.openstack.org/cgit/openstack/ironic/commit/?id=469daf557416c725c9ac32f469b11dcaf3fa6b5a
Submitter: Jenkins
Branch: master

commit 469daf557416c725c9ac32f469b11dcaf3fa6b5a
Author: Devananda van der Veen <email address hidden>
Date: Thu Jan 30 15:57:03 2014 -0800

    Improve handling of invalid input in HashRing class

    HashRing __init__ and get_hosts shouldn't be passed None,
    but if they are, an appropriate error should be raised.

    Change-Id: I139e7410c43bf4ac0f3357797ad767fde9455e36
    Closes-bug: #1274762