Make pbr only a build-time dependency.

This lets you build python-swiftclient packages that don't require pbr
to be installed at all. You would need pbr on the machine running
rpmbuild / debuild, but not on the machines that install the packages.

Unfortunately, this does not make python-swiftclient able to be
installed via pip 0.3.1 on Lucid; you'll need to uninstall the system
python-pip package and install a new pip some other way. Given that
pip < 1.3 doesn't perform SSL certificate validation for pypi (trivial
MITM attack, anyone?), you'd probably want to get a new pip anyway.

Change-Id: I85d4d77aacf094e48d39e48e750594b95dbc7af0
Merge "Make pbr only a build-time dependency."