Select an authz module using setuptools' entry points mechainsm.

Instead of having each authz package install an invirt.authz module,
have them install modules under their own namespace.

In their setup.py, they should indicate that their authz module
provides a unique name within the invirt.authz entry point group.

The new invirt.authz module (part of invirt-base) then gets a name
from the configuration and uses that to find the module.

svn path=/trunk/packages/invirt-base/; revision=2989