set_override method can't hande parameter override=None when enforce_type=True

[ChangBo Guo(gcb)]

Method set_override will check parameter override's type when
enforce_type=True, If parameter override is None, it doesn't
work for some types like IntOpt, or doesn't work properly for StrOpt.
For IntOpt, it raises TypeError, and for StrOpt, it converts None to
string 'None', that's not we expect.

so need skip the parameter override=None for all types.

[ChangBo Guo(gcb) (glongwave)]

code review:
https://review.openstack.org/#/c/243001/

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/243001
Committed: https://git.openstack.org/cgit/openstack/oslo.config/commit/?id=95ba1c4795d013e6ace0c0407a2be03b43d09012
Submitter: Jenkins
Branch: master

commit 95ba1c4795d013e6ace0c0407a2be03b43d09012
Author: ChangBo Guo(gcb) <email address hidden>
Date: Mon Nov 9 17:02:43 2015 +0800

    Allow method set_override with parameter override=None for all types

    Method set_override will check parameter override's type when
    enforce_type=True, If parameter override with value None, it doesn't
    work for some types like IntOpt, or doesn't work properly for StrOpt.
    For IntOpt, it raise TypeError, and For StrOpt, it convert None to
    string 'None', that's not we expect.

    Closes-Bug: #1515230
    Change-Id: I42ceeccf59111057b5a92c849c9bb5f83255b324