Work around atexit bug in python < 2.7.4

In python < 2.7.4, a lazy loading of package `pbr` will break
setuptools if some other modules registered functions in `atexit`.
solution from: http://bugs.python.org/issue15881#msg170215

This problem originally appeared in stevedore before the library
was adopted by Oslo
(https://github.com/dreamhost/stevedore/commit/74a719b11781cc93d60cff1b0a01de3e4e780a1d)

Change-Id: Icd412ca8435dca4b0f3b4075eca39b903dde6d27
Merge "Work around atexit bug in python < 2.7.4"