Fix use the fact that empty sequences are false

[Liusheng]

For sequences(strings, lists, tuples), in order to consistent with pep8 recommendations, 'if len(seq)' should be replaced with 'if seq', and 'if not len(seq)' should be replaced with 'if not seq'.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/64768

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/64768
Committed: https://git.openstack.org/cgit/openstack/ceilometer/commit/?id=00f44590859610c9f0ceea70e30f82c288f3e468
Submitter: Jenkins
Branch: master

commit 00f44590859610c9f0ceea70e30f82c288f3e468
Author: ls1175 <email address hidden>
Date: Fri Jan 3 11:03:27 2014 +0800

    Fix use the fact that empty sequences are false

    For sequences(strings, lists, tuples), in order to consistent with
    pep8 recommendations, 'if len(seq)' should be replaced with 'if seq',
    and 'if not len(seq)' should be replaced with 'if not seq'.

    Change-Id: Ie7b6afbde07bb57a05b36d92a079c5355945b262
    Closes-Bug: #1265688