PEP8 type comparison

[Zhongyue Luo]

According to PEP8,

Programming Recommendations
    - Comparisons to singletons like None should always be done with
      'is' or 'is not', never the equality operators.

    - Don't compare boolean values to True or False using ==
        Yes: if greeting:
        No: if greeting == True:
        Worse: if greeting is True:

Therefore, all comparisons of value XXX to None
should be done by 'XXX is None' or 'XXX is not None'.

All comparison of value XXX to True or False
should be done by 'if XXX' or 'if not XXX'

The "== None", "== True", "== False", and "!= None" in sqlalchemy's
where(), or_(), filter(), and_(), and select() functions should not be changed.

[Zhongyue Luo (zyluo)]

Comparisons in sqlalchemy filter's should not be changed this way.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/2712

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/2712
Committed: http://github.com/openstack/nova/commit/88ccade9d5700db881f2ffc53e4a48a76e92c2db
Submitter: Jenkins
Branch: master

commit 88ccade9d5700db881f2ffc53e4a48a76e92c2db
Author: lzyeval <email address hidden>
Date: Sat Dec 31 12:23:56 2011 +0800

    PEP8 type comparison cleanup

    Fixes bug #910295

    The None, True, and False values are singletons.

    All variable *comparisons* to singletons should use 'is' or 'is not'.
    All variable *evaluations* to boolean should use 'if' or 'if not'.

    "== None", "== True", "== False", and "!= None" comparisons in sqlalchemy's
    where(), or_(), filter(), and_(), and select() functions should not be changed.

    Incorrect comparisons or evaluations in comments were not changed.

    Change-Id: I087f0883bf115b5fe714ccfda86a794b9b2a87f7