buildsys: Improve the getBuild function

This actually fixes all the unit tests.

It also makes the code of the function simpler.
model: Remove unnecessary code

Both our dummy buildsystem and Koji already set the "nvr" key on builds:

    >>> from bodhi.buildsys import DevBuildsys
    >>> session = DevBuildsys()
    >>> session.getBuild('bodhi-0.9.7-1.fc19')["nvr"]
    'bodhi-0.9.7-1.fc19'

    >>> import koji
    >>> session = koji.ClientSession("http://koji.fedoraproject.org/kojihub")
    >>> session.getBuild("bodhi-0.9.7-1.fc19")["nvr"]
    'bodhi-0.9.7-1.fc19'